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: [[ https://linear.app/comm/issue/ENG-1247/implement-tunnelbroker-grpc-api-handlers-in-rust-using-tonic | ENG-1247 ]]