[native] restructure gRPC client cargo project as general purpose library crate for native

Summary:
rearranged the contents of this crate so that @marcin and @inka can add their Rust APIs here, rather than in separate crates that all need to be linked in Xcode/Cmake. A subsequent diff will handle renaming the project.

Depends on D5084

Test Plan: cd native/cpp/CommonCpp/grpc/grpc_client && cargo build

Reviewers: jon, ashoat

Reviewed By: jon, ashoat

Subscribers: ashoat, tomek, abosh, marcin, inka

Differential Revision: https://phab.comm.dev/D5104