[identity] Add function to check if device exists

Summary:
Added a simple query to check if Device item exists in database. Limited projection to avoid fetching all columns.

Depends on D10217

Test Plan: Called this for existing primary key and non-existing primary key.

Reviewers: michal, varun

Reviewed By: michal

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10218