[services] Tunnelbroker - Adding anyhow crate to the Cargo dependencies

Summary:
In D4540 comments @varun suggested using anyhow to easily handle errors. It looks like a good and tested playground solution.

To use it in a Tunnelbroker codebase we need to add it to the Cargo.toml dependencies.
Cargo.lock file is also included in this diff.

Related Linear Task: ENG-1704

Test Plan: Successfully built using the cargo build inside the services/tunnelbroker/rust-notifications.

Reviewers: karol, tomek, jon, ashoat

Reviewed By: karol, tomek, jon, ashoat

Subscribers: varun, ashoat, tomek, atul, karol, abosh

Differential Revision: https://phab.comm.dev/D4952