[identity] Use DB conversion helpers from comm-lib

Summary:
This finally resolves ENG-5476.

Used database conversion traits from comm-lib wherever possible. For some places, I introduced the deprecated attribute. This diff could grow too large.

Depends on D10447

Test Plan: Identity compiles and runs. Commtest passes, also played with it manually.

Reviewers: varun, michal

Reviewed By: michal

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10448