Add support for non-unary gRPC calls

Current plugin for gRPC only supports unary method calls (one method call expects one result).

Adding support for other types of calls is also relevant:
- server-side streaming (one request, many responses)
- client-side streaming (multiple requests, at most one response)
- bi-directional streaming, which might be a bit trickier to represent as a transaction (if used as a long-lived communication like a socket).

Please :+1: this issue and/or comment so we can prioritize support for those.