AWS CloudFormation and Lambda - All-in-one cloudformation stack with embedded lambda custom resource. See https://missionimpossiblecode.io/building-a-best-practice-cloudformation-custom-resource-pattern

cloudformation_custom_resource_all_in_one.yml

# Source: https://missionimpossiblecode.io/building-a-best-practice-cloudformation-custom-resource-pattern
Parameters:
  SpecifyVPCToUse:
    Description: >
      DefaultVPC - finds the VPC and configures all of its subnets for you. Otherwise type 
      in the VPC id of a VPC in the same region where you run the template. 
      All subnets and azs of the chosen vpc will be used.
      The VPC and chosen subnets must be setup in a way that allows the runner instances 
      to resolve the DNS name and connect to port 443 on the GitLab instance URL you provide.      
    Default: DefaultVPC
    Type: String
    # While it is tempting to make the above parameter of type "AWS::EC2::VPC::Id" 
    # this prevents automatic discovery and usage of the DefaultVPC.
    # However, if your organization NEVER uses default VPCs or disables them, changing 
    # the type to AWS::EC2::VPC::Id actually improves the user experience because users do not have to 
    # lookup VPC ids in the console.

Resources:
  #Arguments: Vpc-id or "DefaultVPC"
  #Returns: vpc-id, number of subnets and ordered list of subnetids and az ids.  
  # The index of these two return lists are correlated if it is desirable to choose less than the whole list using the CloudFormation function "Select" against both lists.
  VPCInfoLambda:
    Type: 'AWS::Lambda::Function'
    Properties:
      Description: Returns the lowercase version of a string
      MemorySize: 256
      Runtime: python3.8
      Handler: index.handler
      Role: !GetAtt CFCustomResourceLambdaRole.Arn
      Timeout: 240
      Code:
        ZipFile: |
          import logging
          import traceback
          import signal
          import cfnresponse
          import boto3

          LOGGER = logging.getLogger()
          LOGGER.setLevel(logging.INFO)

          def handler(event, context):
              # Setup alarm for remaining runtime minus a second
              signal.alarm((int(context.get_remaining_time_in_millis() / 1000)) - 1)
              try:
                  LOGGER.info('REQUEST RECEIVED:\n %s', event)
                  LOGGER.info('REQUEST RECEIVED:\n %s', context)
                  if event['RequestType'] == 'Delete':
                      LOGGER.info('DELETE!')
                      cfnresponse.send(event, context, "SUCCESS", {
                          "Message": "Resource deletion successful!"})
                      return
                  elif event['RequestType'] == 'Update':
                      LOGGER.info('UPDATE!')
                      cfnresponse.send(event, context, "SUCCESS",{
                          "Message": "Resource update successful!"})
                  elif event['RequestType'] == 'Create':
                      LOGGER.info('CREATE!')
                      request_properties = event.get('ResourceProperties', None)

                      VpcToGet = event['ResourceProperties'].get('VpcToGet', '')
                      ec2 = boto3.resource('ec2')
                      VpcCheckedList = []
                      TargetVPC = None
                      vpclist = ec2.vpcs.all()
                      for vpc in vpclist:
                          VpcCheckedList.append(vpc.id)
                          if VpcToGet == "DefaultVPC" and vpc.is_default == True:
                              TargetVPC=vpc
                          elif vpc.vpc_id == VpcToGet:
                              TargetVPC=vpc

                      if TargetVPC == None:
                        raise Exception(f'VPC {VpcToGet} was not found among the ones in this account and region, VPC which are: {", ".join(VpcCheckedList)}')
                      else:
                        VPCOutput = TargetVPC.id
                        subidlist = []
                        zoneidlist = []
                        subnets = list(TargetVPC.subnets.all())
                        for subnet in subnets:
                          subidlist.append(subnet.id)
                          zoneidlist.append(subnet.availability_zone)
                        subidOutput = ",".join(subidlist)
                        zoneidOutput = ",".join(zoneidlist)
                        if not subnets:
                          raise Exception(f'There are no subnets in VPC: {VpcToGet}')
                        LOGGER.info('subnet ids are: %s', subidOutput)
                        LOGGER.info('zone ids are: %s', zoneidOutput)

                      responseData = {}
                      responseData['VPC_id'] = VPCOutput
                      responseData['OrderedSubnetIdList'] = subidOutput
                      responseData['OrderedZoneIdList'] = zoneidOutput
                      responseData['SubnetCount'] = len(subidlist)
                      cfnresponse.send(event, context, cfnresponse.SUCCESS, responseData)                                  

              except Exception as err:
                  AccountRegionInfo=f'Occured in Account {context.invoked_function_arn.split(":")[4]} in region {context.invoked_function_arn.split(":")[3]}'
                  FinalMsg=str(err) + ' ' + AccountRegionInfo
                  LOGGER.info('ERROR: %s', FinalMsg)
                  LOGGER.info('TRACEBACK %s', traceback.print_tb(err.__traceback__))
                  cfnresponse.send(event, context, "FAILED", {
                      "Message": "{FinalMsg}"})

          def timeout_handler(_signal, _frame):
              '''Handle SIGALRM'''
              raise Exception('Time exceeded')

          signal.signal(signal.SIGALRM, timeout_handler)


  #Custom Function IAM Role Declaration
  CFCustomResourceLambdaRole:
    Type: AWS::IAM::Role
    Properties:
      AssumeRolePolicyDocument:
        Version: "2012-10-17"
        Statement:
          - Effect: "Allow"
            Principal:
              Service:
                - "lambda.amazonaws.com"
            Action:
              - "sts:AssumeRole"
      Policies:
        - PolicyName: "lambda-write-logs"
          PolicyDocument:
            Version: "2012-10-17"
            Statement:
              - Effect: "Allow"
                Action:
                  - "logs:CreateLogGroup"
                  - "logs:CreateLogStream"  
                  - "logs:PutLogEvents"
                Resource: "arn:aws:logs:*:*"
        - PolicyName: "describe-vpcs-and-subnets"
          PolicyDocument:
            Version: "2012-10-17"
            Statement:
              - Effect: "Allow"
                Action:
                  - "ec2:DescribeVpcs"
                  - "ec2:DescribeSubnets"
                Resource: "*"

  #Calling Function to Retrieve Data
  LookupVPCInfo:
    Type: Custom::VPCInfo
    Properties:
      ServiceToken: !GetAtt VPCInfoLambda.Arn
      VpcToGet: !Ref SpecifyVPCToUse

Outputs:
  VPCZoneIdentifier:
    Description: VPCZoneIdentifier
    Value: !GetAtt LookupVPCInfo.OrderedSubnetIdList
  AvailabilityZones:
    Description: AvailabilityZones
    Value: !GetAtt LookupVPCInfo.OrderedZoneIdList
    # Value: !Split [",",!GetAtt LookupVPCInfo.OrderedZoneIdList]
    # Export: 
    #   Name: