Make service and interceptor activators public

[JamesNK]

Fixes #671

[davidfowl]

@JamesNK might be worthwhile adding a test with a dummy implementation (like a fake container).

[JunTaoLuo]

Will you be taking any action on the other asks of the issue? Making these types public was listed as the first step, but there's also asks for making it non-generic. We should clarify either way.

Edit: NVM I see the rework happening in https://github.com/grpc/grpc-dotnet/pull/669/files.

[jtattermusch]

I'm ok with this, but if we do that, we are committing to API stability for the interfaces, so I'd like us to think twice. @JamesNK @JunTaoLuo

[JamesNK]

The API shape has been debated a lot and I think this is the best solution.

I'll get the original requester of this feature to double check it works for them before the package is finalized.