[services] Tunnelbroker - Expose isConfigParameterSet function to the Rust

Summary:
This function exposes the isConfigParameterSet function from the C++ codebase to the Rust by using the CXX function.
This function will be used in the next D6253 diff to retrieve the boolean status if the config file flag.

Linear task: ENG-2642

Test Plan: Adding the skip_authentication flag to the [sessions] section of the config file results in a true return from the isConfigParameterSet('sessions.skip_authentication') function call from the Rust codebase.

Reviewers: jon, bartek

Reviewed By: jon, bartek

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D6252