[services] Backup - Rust lib - Add is initialized

Summary:
Depends on D4861

Adding a function that checks whether the client's been initialized

Test Plan: test plan in D4870

Reviewers: varun, tomek, ashoat

Reviewed By: varun, tomek, ashoat

Subscribers: ashoat, tomek, atul, abosh

Differential Revision: https://phab.comm.dev/D4884