[RFC] SCIM support for auto-provisioning

We should look into System for Cross-domain Identity Management support for auto-provisioning.

I've seen many requests for "oAuth2" that are in fact requests for "centrally managing identities elsewhere". Well, guess what? Even if we enable account creation on first use (like we did with PROXY_AUTH_CREATE) it will not handle their lifecycle (when accounts get disabled or renamed - off-boarding).

This has been briefly discussed on #mailu-dev and the current consensus of those that said they would use OIDC is that it is overkill. This issue is open to track interest, add a thumb up if you require it (with or without OIDC).

#783 #823 #1984 #2378 #2575 #3675