[identity] Add function to update social proof

Summary:
Whitepaper 5.2 says that the social proof needs to be updated for wallet users during the backup restore RPC.
Added a function which updates the social proof value in DDB.

Depends on D13224

Test Plan:

• Registered a wallet user
• Generated new social proof and provided it to the function (manually)
• Verified that DDB contains new social proof value
• Verified that conditional check fails for password users (no previos social proof present)

Reviewers: varun, will

Reviewed By: varun, will

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13225