[services] Tunnelbroker - Adding a .rustfmt.tomlto the project root

Summary:
To make Rust formatting consistent with the codebase this file adds tab spacing and maximum width rules for the Rust formatting cargo fmt.
We are placing the .rustfmt.toml file at the root of the project. According to the documentation, we can use the project-wide file at the root of the project (parent directory).

The rustfmt.toml files form the sub-directories (services/commtest, services/identity, native/cpp/CommonCpp/CryptoTools/opaque-ke-cxx) were removed.

Linear task: ENG-1330

Test Plan: Running cargo fmt from the Rust workspace directory formats the code according to the standard.

Reviewers: karol-bisztyga, varun, jonringer-comm, ashoat

Reviewed By: varun, ashoat

Subscribers: ashoat, palys-swm, Adrian, atul, karol-bisztyga, yayabosh

Differential Revision: https://phab.comm.dev/D4553