[web] add gRPC client wrapper

Summary: we want the client methods to match what we have on native in commRustModule. also, this wrapper class lets us hide some other implementation details that the caller doesn't need to worry about (e.g. version interceptor, auth vs unauthorized client)

Test Plan:
successfully called local identity service by setting localhost socket address in keyserver/secrets json file

checked that if config is absent, the correct default is used depending on NODE_ENV value

Reviewers: inka, tomek, ashoat

Reviewed By: tomek, ashoat

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D10244