[identity] Send RETRY for transaction conflicts

Summary:
Part of ENG-9427.

• Added a function that checks if DDB error is transaction conflict (we already have one but this one is more opinionated)
• Added a case to handle_db_error to send retry tonic message instead of unexpected error

Depends on D13567

Test Plan: Simulated TransactionConflictException DDB error and checked gRPC status code in a UI client

Reviewers: varun, will

Reviewed By: will

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13568