feat: Support adding events to existing functions (take 2)

[Jacco]

Issue #866:
Support adding events to existing functions

Description of changes:

• class SamEventsResource(SamResourceMacro) to share events handling with SamFunction, SamApplication and SamFunctionReference
• class SamFunction now inherits from SamEventsResource
• class SamStackFunctionReference(SamResourceMacro) helper function reference for sar
• class SamFunctionReference(SamEventsResource) introduced for use as new sam resource that supports events
• class SamApplication now inherits from SamEventsResource so events can be used on it

Description of how you validated changes:
Added two translator tests and two integration tests

Checklist:

• Write/update tests
• make pr passes
• Update documentation
• Verify transformed template deploys and application functions as expected

Examples?

Please reach out in the comments, if you want to add an example. Examples will be
added to sam init through https://github.com/awslabs/aws-sam-cli-app-templates/

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

[jfuss]

@Jacco Just wanted to first say, thank you for spending the time on this. Given how old this is and conflicting files, I am going to close it.

I think this still could be powerful but it is unclear how we will want to achieve this. We are looking into what is next for SAM (#2319), which I think this could be a good candidate to make sure we solve (at the very least) within that context/issue.