gRPC client and service without Protobuf

Although solved partially with #1703, but it’d be nice if we can let our users use an alternative marshaller instead of Protobuf while still using gRPC-Java API, as demonstrated in this blog post by @carl-mastrangelo: https://grpc.io/blog/grpc-with-json/

@anuraaga told me it is already possible on the server side since we allow a user to mount any BindableServices, but client-side needs some more work like exposing ArmeriaClientCall to a user.

I’d also be nice if we provide some working example, which is similar to grpc-gateway with some twist, e.g. using Jackson’s JsonNode tree as a message.

(Initially requested by @okue)

[trustin]

Eventually like this:

ClientCall call = Clients.builder(...).....build(ClientCall.class);

[anuraaga]

It shouldn’t, we’d expose a new accessor using the gRPC interfaces without exposing classes.