[identity] check if wallet address is reserved in LoginWalletUser RPC

Summary:
if the user already exists on the keyserver but is logging into the identity service for the first time, we should send back a specific error code instructing the client to get a signed message confirming their wallet address and user ID.

Depends on D9678

Test Plan: called the LoginWalletUser RPC with a reserved wallet address and received the expected error status code and message

Reviewers: michal, bartek, jon

Reviewed By: michal

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9679