[identity] add RPC for logging in users with reserved wallet addresses

Summary:
wallet users on ashoat's keyserver today will have to claim their address when they first auth with the identity service by calling this special RPC.

while implementing the new RPC i found some places in our code base that needed refactoring. if it's difficult to review i can split it out.

Test Plan: added my wallet address to the reserved usernames table. then successfully claimed it using the new RPC

Reviewers: bartek, michal, jon

Reviewed By: bartek, michal

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9678