WIP: Implemented support for CORS in start-api command

[devoto13]

Issue #, if available:

#323

Description of changes:

This adds a basic support for CORS in start-api command. Currently it only understands format specified in SAM and doesn't understand explicit definition with x-amazon-apigateway-integration with type=mock. I believe that this should solve most of the use cases and support for the explicit specification can be added at the later stage as it is a more complex task.

At this stage I'm looking for the confirmation that the direction is right and if so I'll proceed with a cleanup, tests and documentation.

Checklist:

• Write Design Document (Do I need to write a design document?)
• Write unit tests
• Write/update functional tests
• Write/update integration tests
• make pr passes
• Write documentation

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

[devoto13]

Just a short updated on this. I've used cors property of the Route class, because it was already there, but after some thinking it seems that the better idea is to remove the property and instead introduce two subclasses for Route: one for Lambda integration and another one for the CORS response (which is essentially a quite limited type=mock integration, which can later be extended).

Then the implementation will add a new OPTIONS route for every unique path and if this route is hit, it will respond with the CORS headers specified in the SAM.

[devoto13]

I switched to another project and won't have time to move this forward in the near future, so closing the PR. There is a decent workaround described in #323 (comment).