[services] Tunnelbroker - Add env_logger to the Rust library

Summary:
This diff introduces adding and using of env_logger crate to configure the logging level via environment variables.

We already have the same approach in the backup service (D5124).

Test Plan: Start tunnelbroker with the different logging levels using environment variables.

Reviewers: jon, tomek, ashoat

Reviewed By: jon, tomek, ashoat

Subscribers: ashoat, tomek, atul, karol, abosh

Differential Revision: https://phab.comm.dev/D5182