Codegen base class for services to extend

[ejona86]

We should allow adding new methods to a service without breaking existing service implementations. The best way to do that seems to have service implementations extend a base class that responds with UNIMPLEMENTED for all methods. The class would also make creating fake/mock services during tests easier.

A similar problem can happen for stubs in tests. We may need something there as well.

See also grpc/grpc#5371

Edit: IOW, remove all interfaces from the generated code, since adding a method will break existing implementations of that interface.

[louiscryan]

Will pick this up...

[ejona86]

As part of this we'll need to update the webpage docs.