[identity] Verify prekeys in RefreshUserPrekeys RPC

Summary:

• Adds prekey signature verification in RefreshUserPrekeys RPC (ENG-10381)
• Some helper APIs to get notif signing public key

Depends on D14459

Test Plan: Commtest fails without D14459 which introduced valid signatures

Reviewers: kamil

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D14460