Support JSON-encoded request/response data from gRPC server

[arya2]

Motivation

We want to support JSON-encoded request/response data in the new gRPC server.

This will be more important if we choose to migrate existing JSON RPC methods in zebra-rpc to a gRPC server.

Possible Design

See the json-codec example code:

• https://github.com/hyperium/tonic/blob/master/examples/build.rs#L44
• https://github.com/hyperium/tonic/tree/master/examples/src/json-codec