[identity] Add function to delete all user devices data

Summary:
Resolves ENG-5120.

Depends on D10221

Test Plan: Called DeleteUser RPC and verified with awslocal dynamodb scan that no rows were left in the identity-devices table with given userID.

Reviewers: varun, michal

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10246