Here is what this code does:
1. Create a list of pre-signed urls - the number depends on the chosen chunk size (e.g. 8MB) 
2. Upload a sample file to S3 using pre-signed URLs. In the real-world scenario we would generate the link on the server side, and upload data using these URLs on the client side.
3. Complete the multipart upload. Again this one would be executed on the server as well. Additionally you may request object size on the server side after completion, and return the size to the client,
