Expose ServerCallStreamObserver to server-side stubs

### Is your feature request related to a problem?
At the moment, service methods of a base class for the server implementation (generated by `compiler/src/java_plugin/cpp/java_generator.cpp`'s `PrintAbstractClassStub`/`PrintStub`) have a `responseObserver` formal parameter of `io.grpc.stub.StreamObserver` type, despite that in runtime it is always an instance of `io.grpc.stub.ServerCallStreamObserver`, and using it requires a typecast, which is explicitly stated in its javadoc:

> An instance of this class is obtained by casting the {@code StreamObserver} passed as an argument to service implementations.

While this is not a disaster, and just takes some time to figure out the intended way of using server-side APIs (such as `setCompression(String)`) and can be improved even further by adding more details of this in the documentation, it still
1. Requires a boilerplate type casting inside of a service method (I currently see 30+ expressions similar to`final ServerCallStreamObserver responseObserver =(ServerCallStreamObserver) observer;` in grpc-java's own test code).
2. Does not give a clear understanding of an actual argument's type by simply looking at a generated code.
3. Potentially opens a way for an implementation issue where a non-`ServerCallStreamObserver` instance can be passed in runtime.

### Describe the solution you'd like
Since server call stream observer (along with a client one) [is already considered a stable and non-experimental feature](https://github.com/grpc/grpc-java/issues/1788), it may be practical to use a refined type explicitly in generated stubs (instead of a `io.grpc.stub.StreamObserver` interface) to give a developer a clear and robust way of calling relevant APIs and leave less room for an implementation error/deficiency.
On a side-note: one more potential improvement is to make it an interface instead of abstract class once `io.grpc.stub.CallStreamObserver` [becomes a non-experimental feature](https://github.com/grpc/grpc-java/issues/8499) to explicitly mark it as a part of gRPC API and allow for slightly easier mocking.

### Describe alternatives you've considered
Continue the current approach with a certain documentation improvement, e.g. adding a note on the usage of refined abstract classes to [Generated-code reference article](https://grpc.io/docs/languages/java/generated-code/).

Beginne in compiler/src/main/java_plugin/cpp/java_generator.cpp, insbesondere bei PrintAbstractClassStub und PrintStub, und untersuche, wie serverseitige responseObserver-Parameter generiert werden. Sieh dir den Referenzartikel zum generierten Code und die bestehende Verwendung in Serverimplementierungen an. Als abgeschlossen gilt die Aufgabe, wenn ein abgestimmter Ansatz konsistent in generierten Stubs implementiert wurde und die Dokumentation bei Bedarf aktualisiert ist.

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