[services] Rust Integration - Backup - Rust - Add converting functions

Summary:
Depends on D5031

Adding helper functions to convert between rust's String and c++'s char* both ways.

Test Plan: backup builds successfully

Reviewers: jon, tomek, varun, max

Reviewed By: tomek

Subscribers: ashoat, tomek, atul, abosh

Differential Revision: https://phab.comm.dev/D5034