[keyserver] update login_user_wallet function

Summary:
updated the login_user_wallet function to use the new API specified in identity_client.proto

Depends on D7356

Test Plan: this will get tested later in the stack once i can rebase on jon's changes

Reviewers: jon, ashoat

Reviewed By: jon

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D7361