[tunnelbroker-client] Adding Tunnelbroker tonic client initialization function

Summary:
This diff introduces an initialize_client function that initializes the Tonic client instance and returns a client channel for it. The client instance from this function will be passed as an argument into the publish and consume functions in the follow-up diffs.

Linear task: ENG-2728

Test Plan: Running of run nix develop --accept-flake-config -c bash -c "cargo test" will successfully build the library.

Reviewers: jon, varun

Reviewed By: jon, varun

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D6281