[shared] [identity] [2/n] Add identity_search_messages to shared and use in identity

Summary:
This introduces identity_search_messages into shared for usage by identity's prefix search websocket services and commtest (in later diffs).

Depends on D10802

Test Plan: cargo fmt and build. Successfully running identity service

Reviewers: varun, bartek

Reviewed By: varun, bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10804