Support extensions as nested annotations for extensible models

[MikeEdgar]

Fixes #339
Fixes #387

[MikeEdgar]

We may want to add some additional TCK tests for some random selection of annotations.

[Azquelt]

Should we add the extensions array to the @Content annotation as well? As far as I can make out, it represents a key and value pair from the a content map. The value corresponds to a MediaType model object which is extensible.

[MikeEdgar]

Should we add the extensions array to the @Content annotation as well?

Yes, thanks for catching it.

[Azquelt]

Should we add extensions to @APIResponses? I think that's allowed even though it's a repeatable container for @APIResponse as long as it has a default value and the APIResponses model object is extensible.

[Azquelt]

I'm unsure whether we should add it to @APISchemaResponse or not. I think that corresponds to a Response but it's also already a shorthand and users who want to add extensions could use the full @APIResponse instead.

[MikeEdgar]

it's also already a shorthand and users who want to add extensions could use the full @APIResponse instead

That's my thinking as well.

[Azquelt]

I've finished my review and I'd be happy to approve once the comments above are addressed.

[Azquelt]

Changes look good but spotted one issue in the test.