fix: implemented change to always use __ as prefix and suffix for parameter names generated for a LambdaFunction.

[ashishdhingra]

Issue #, if available: #1977

Description of changes:
Implemented change to always use __ as prefix and suffix for parameter names generated for a LambdaFunction.

Debugging Steps:
Referred article https://nicksnettravels.builttoroam.com/debug-code-gen/ to debug the project (needed to understand the code flow for source generator):

• Added below reference of Amazon.Lambda.Annotations.SourceGenerator project in the target application project:

  <ItemGroup>
    <ProjectReference 
  		Include="..\aws-lambda-dotnet\Libraries\src\Amazon.Lambda.Annotations.SourceGenerator.csproj"
  		OutputItemType="Analyzer"
  		ReferenceOutputAssembly="false" />
  </ItemGroup>

• Uncommented the below code in Generator constructor:

  #if DEBUG
    if (!Debugger.IsAttached)
    {
        Debugger.Launch();
    }
  #endif 

We would need to clean solution, close Visual Studio, reload project solution. It would prompt to select Visual Studio instance for debugging. If selecting current instance, it would have dialog looking for change in source files as Generator() constructor is debugged.
Thereafter, set breakpoint in Execute() method and expand Dependencies > Analyzers > Amazon.Lambda.Annotations.SourceGenerator. It will try to inspect target application project and execute source generator logic.

This PR also includes the following changes:

• Executed command autover change --project-name "Amazon.Lambda.Annotations" -m "fix: implemented change to always use __ as prefix and suffix for parameter names generated for a LambdaFunction." to create the change file using ONLY Amazon.Lambda.Annotations project. Per @normj, there is a single NuGet package with the both Amazon.Lambda.Annotations.dll for runtime and Amazon.Lambda.Annotaitons.SourceGenerator.dll run by the Roslyn compiler. The csproj file for Amazon.Lambda.Annotations.SourceGenerator. Refer existing commit.

• Updated integration test snapshots which would be changed as a result of this change.

• Updated existing test to include the above scenario.

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