Support for Lambda Payload version 2.0

[realharry]

Is your feature request related to a problem? Please describe.

AWS recently announced GA for "HTTP API" for API Gateway, which supports a newer JSON format for Lambda Payload. It'll be nice if we can use aws-lambda-go with HTTP API/Lamda payload format 2.0.

Describe the solution you'd like

An optional flag to use version 2 vs version 1 formats.

Describe alternatives you've considered

N/A

Additional context

Relevant blog post: https://aws.amazon.com/blogs/compute/building-better-apis-http-apis-now-generally-available/

[shashank-sachan]

+1
I was also looking for the new payload support like requestContext.routeKey and other key pairs.

[bmoffatt]

Implemented by #274