[identity] Add function to retrieve user's devices

Summary:
This function retrieves all Device rows for given user. This is a counterpart of get_keys_for_user function in "legacy" schema.

Depends on D10216

Test Plan: Added a few test rows (with both device- and devicelist- prefixes) to the identity-devices table and verified that the function returns only the device- items.

Reviewers: michal, varun

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10217