[identity] Add exponential backoff to refresh_user_prekeys

Summary:
Part of ENG-9427.
Added exponential backoff retry counter to RefreshUserPrekeys RPC

Depends on D13568

Test Plan: It's hard to do that on localstack, but I was able to reuse test plan from D13356 to block DB item with transactions and then confirmed that refresh_user_prekeys also retries.

Reviewers: varun, will

Reviewed By: varun, will

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13569