[identity] Remove deprecated conversion traits

Summary:
After legacy device list code was removed, the conversion traits were no longer used. This removes them and replaces last usages with the proper calls from comm-lib. This will also simplify the social proof fix (next diffs).

I had to add implementation for optional attributes to comm-lib, since it was missing.

Depends on D10701

Test Plan:

• Identity integration tests
• Unit tests for comm-lib to test optional attributes

Reviewers: michal, varun

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10762