[keyserver] Introduce fetchNativeCookieIDsForUserIDs

Summary:
See code comment. We'll need this for ENG-3241.

Depends on D6982

Test Plan: I tested in Node.js REPL

Reviewers: varun, atul, jon

Reviewed By: varun

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6983