[Feature Request] Add custom user type

[lamoboos223]

Description:

instead of having the default user types, we should allow admins to register their user type for their users for different use cases, for example in the directive auto_accept_invites in homeserver.yaml we can set it to accept invites from users with certain user type.

[reivilibre]

This was actually implemented shortly after your ticket: #18456 :-)