AEP-156: provide a way to differentiate singletons and collections

[toumorokoshi]

Today, singletons and collections same the same resource path. For example, a GET on /clusters/{cluster}/instance may be either a collection, or a singleton. Although instance is singular and perhaps a human can determine that it is a singular, a machine cannot reliably determine that the path is a singleton.

This is problematic for things like the OpenAPI linter, which must have some heuristic to determine if the path is a collection, and therefore should have a different set of linter rules applied to it.

Some ideas floated around include:

• Doing something similar to the IBM styleguide, where it looks for an array of objects and some field that is only common on collections (e.g. skip or offset).
• Adding an annotation (x-aep-collection) to the OpenAPI description.
• Using a media content type to specify that the response is a collection (e.g. application/collection+json.

CC @mkistler @hudlow