support Lambda Payload version 2.0 in HTTP API

[stevemao]

Lambda proxy integrations in API Gateway Payload version 2.0 has a slightly different format. https://medium.com/@lancers/amazon-api-gateway-explaining-lambda-payload-version-2-0-in-http-api-24b0b4db5d36
https://aws.amazon.com/blogs/compute/building-better-apis-http-apis-now-generally-available/