[services] add public key and username fields to .proto file

Summary:
The identity service maintains a mapping from userID → [user_info, public_key], and also user_info → userID. user_info is a username for password auth and a wallet address for wallet auth. The public_key is used to log users into keyservers. Users should send the public_key as part of PAKE registration or Wallet login.

Note: We already get the wallet address in the siweMessage as part of the WalletLoginRequest

Test Plan: Checked syntax with linter, identity service build succeeded

Reviewers: ashoat, tomek, karol, max

Reviewed By: ashoat, tomek

Subscribers: adrian, atul, abosh

Differential Revision: https://phab.comm.dev/D4613