Expose Server.SERVER_CONTEXT_KEY so that users can implement a Server

It seems like a valid goal to want to implement (extend) a `Server` - there are only a few implementations in grpc-java and I can imagine quite a few alternatives that I wouldn't expect the grpc-java community to want to support. But if you try it you find that the reflection service doesn't work (and therefore I assume potentially other services) because it accesses the current `Server` via the `Server.SERVER_CONTEXT_KEY`. If you don't have access to that key you can't set up the context for the reflection service. There is an escape hatch via `InternalServer.SERVER_CONTEXT_KEY` but that is clearly documented as "not for use outside grpc-java" and annotated with `@Internal`.