[web] client method for login password user

Summary: this method handles all of the logic around session ids (for sticky sessions on identity service) and OPAQUE. we return a string because that's what we do in CommRustModule, and we would like to have a single interface for the two clients.

Test Plan: called loginPasswordUser from root.js and got back a string, from which userID and access token could be parsed

Reviewers: tomek, inka, ashoat, kamil

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D10455