[WIP] ldap-sync: Scaffold package and reconciliation core

See https://phabricator.wikimedia.org/T420691#11817798

Placeholder: Will fill out MR body later (also need to do some commit cleanup) but just opening this now for visibility's sake.

Current state: exercised a subset of codepaths locally, but i'll need to do a followup test on staging from a wmf host to be able to fully test the functionality before we're reading to proceed to actually deploying in k8s via cronjob (or alternatively, airflow)

Bug: T420691