Example of using a server as the GrpcService in a client

[kjvalencik]

I wanted to write some end-to-end tests for code that uses a tonic client. I wanted to have a mock server without needing to mock the gRPC client.

I started with the mock example which uses a duplex stream for in-process I/O, but that felt overly complicated. I then started looking at the GrpcService trait and creating an implementation that would use my mock server directly.

While writing that implementation, I discovered that tonic servers already implement this trait. You can simply pass a server to a client and it works.

let server = GreetServiceServer::new(Greeter::default());
let client = GreetServiceClient::new(server);

I couldn't find any examples of this pattern in the repo and it's very handy for testing.

Would you be willing to accept a PR to add an example demonstrating this? Would it be preferable to replace the mock example with this pattern?