[native] more refactoring of the native rust library

Summary:
remove the header files after build so they don't appear in the devs' git status. also rename grpc_client -> native_rust_library and move it out of the cpp folder. we're renaming the cargo project because it will be more than a gRPC client library going forward.

Depends on D5104

Test Plan: cleaned the Xcode build folder, removed all build artifacts from git repo with cargo clean, deleted the header files, and then ran Xcode build

Reviewers: ashoat, jon

Reviewed By: ashoat

Subscribers: marcin, inka, ashoat, abosh

Differential Revision: https://phab.comm.dev/D5168