[keyserver] add loginUserWallet function

Summary: expose a new client function to double-write siwe login info to the identity service

Test Plan:
this gets tested in the next diff where we call loginUserWallet from user-responders.js

Depends on D6942

Reviewers: ashoat, jon, bartek

Reviewed By: ashoat, jon

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D6943