[services] Backup - Blob Get Client - Pass rust string instead of const char*

Summary:
Depends on D5097

Raised in https://phab.comm.dev/D5035#148669

We can construct rust::Strings in c++ instead of passing const char*s from c++ to rust.

Test Plan: Services build, tests pass.

Reviewers: jon, tomek, max, varun

Reviewed By: jon, tomek

Subscribers: ashoat, tomek, atul, abosh

Differential Revision: https://phab.comm.dev/D5099