[services][identity] db client method to get user public key

Summary:
first we use the user ID to get the associated item from the users table in DDB. then we try to find the device ID in the item's devices map, and if successful, return the device's user public key.

Depends on D5568

Test Plan: tested in the next diff when the getUserPublicKey RPC is callable

Reviewers: tomek, bartek, jon

Reviewed By: tomek

Subscribers: ashoat, atul, abosh

Differential Revision: https://phab.comm.dev/D5569