Refactor get user identifier to return identity with farcaster id

Summary:
This includes a new identity struct with a farcaster id field which we refactor get_user_identifier to return. This follows discussion from
https://linear.app/comm/issue/ENG-7759/modify-finduseridentity-to-take-list-of-userids-and-include-fid.

Depends on D11859

Test Plan: cargo check

Reviewers: bartek, kamil

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11951