Support for Lambda@Edge

## 🚀 Feature Proposal

Add support for Lambda@Edge payload, [serverless-express supports it](https://github.com/vendia/serverless-express).

This is pretty much just understanding its input/output, I believe no major changes would be required as it's somewhat similar to the regular integration.

## Motivation

To have a unified interface between API Gateway and Lambda@Edge.

## Example

Same as above.