Virtual thread friendly client proxy generation

[vietj]

Provide a virtual thread friendly client proxy generation.

• unary/unary interactions
• unary/multi interactions
• multi/unary interactions
• multi/multi interactions
• proxy API design
• documentation

Stream<Messages.StreamingOutputCallResponse> stream = client.streamingOutputCall_sync(request);
stream.forEach(msg -> ...);
HelloReply reply = client.sayHello_sync(request);