Authenticate against LDAP on demand without syncing the whole directory

## What

authenticate against a large LDAP/AD on demand without syncing/storing the entire directory (and password hashes) locally.

## Want

- Authenticate against a large LDAP/AD on demand without syncing/storing the entire directory (and password hashes) locally

## Ideas this came from

- [#5764 Fetch LDAP user on demand and without storing password hashes](https://github.com/goauthentik/authentik/issues/5764) — Yes, we would like to use authentic with a pretty huge LDAP backend. We do not want to sync that entire LDAP into authentic for various reasons. Aside from that
- [#8080 Is LDAP federation without user sync possible?](https://github.com/goauthentik/authentik/issues/8080) — Describe your question/ I'm trying to setup a LDAP federation with large scale LDAP (AD) server consists of 10k+ users accounts. I've setup a federation but cou

Start by reviewing related issues #5764 and #8080, then trace the existing LDAP/AD authentication flow. Done means authenticating users on demand against a large directory without synchronizing the full directory or storing password hashes locally.

Written by the indexing model from the issue text.