Modules.cs

public static class ServiceCollectionExtensions
{
    internal static readonly string ModulesKey = "Modules";

    public static IServiceCollection AddModules(this IServiceCollection serviceCollection, HostBuilderContext builderContext)
    {
        var modulesSection = builderContext.Configuration.GetSection(ModulesKey);
        foreach (var moduleName in modulesSection.GetChildren())
        {
            var type = Type.GetType(moduleName.Value, true, false);
            if (type == null)
                throw new InvalidOperationException($"Module name {moduleName.Value} was not resolved.");

            var module = (IConfigurationModule)Activator.CreateInstance(type)!;
            serviceCollection.AddModule(builderContext, module);
        }

        return serviceCollection;
    }

    public static IServiceCollection AddModule(this IServiceCollection serviceCollection, HostBuilderContext builderContext, IConfigurationModule module)
    {
        module.ConfigureServices(builderContext, serviceCollection);
        return serviceCollection;
    }
}

public interface IConfigurationModule
{
    void ConfigureServices(HostBuilderContext context, IServiceCollection services);
}

// appsettings.json
{
 "Modules": ["ClassName, LibName"]
 }