[services] Tunnelbroker - Adding of constants.rs file

Summary:
Following the current codebase approach, this diff introduces the constants.rs file which will contain global constants.
The currently added constants are:

• GRPC_TX_QUEUE_SIZE - The gRPC stream queue size. Used in D5529.
• GRPC_PING_INTERVAL_MS - Client pinging interval. Used in D5529.

Linear task: ENG-1247

Test Plan: Passing the build tests in D5529.

Reviewers: jon, marcin, tomek

Reviewed By: jon, tomek

Subscribers: ashoat, tomek, atul, abosh

Differential Revision: https://phab.comm.dev/D5526