Fix issue with Source generator for Lambda functions that return void

[normj]

Description of changes:
For Amazon.Lambda.Annotations if a method with the LambdaMethod attribute has its return type set to void the source generator would generate invalid code.

For example if you had a Lambda function like this

public class VoidExample
{
    [LambdaFunction()]
    public void VoidReturn(string text, ILambdaContext context)
    {
        context.Logger.LogLine(text);
    }
}

The generated code that wraps this call would previously look like this with an invalid return statement when calling this void method.

public class VoidExample_VoidReturn_Generated
{
    private readonly VoidExample voidExample;

...

    public void VoidReturn(string text, Amazon.Lambda.Core.ILambdaContext __context__)
    {
        return voidExample.VoidReturn(text, __context__);
    }

...
}

This fix removes the invalid return statement. The reason this code was happening is the code was treating methods that return void and Task as the same via the ReturnVoidOrTask property. Since it is not safe to treat void and task the same I replaced this property with ReturnVoid and ReturnTask forcing users to think what is the right thing to do in the context.

Ignore the LambdaFunctionTemplate.cs file because that is generated from the LambdaFunctionTemplate.tt file.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[ganeshnj]

Should this exist?

serverless.template must have the new function.

[normj]

Removed

[hounddog22030]

@normj - is this related: #1200

thanks