[protos] Refactor user Identity message type

Summary:
Addresses ENG-7532.

• Replaced oneof with username and optional EthIdentity
• Replaced social_proof with separate siwe message and signature fields

Depends on D11546

Test Plan: Registered password and wallet user. Called FindUserIdentity for both and verified the respponses.

Reviewers: ashoat, varun, will

Reviewed By: ashoat, varun

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D11547