[#41299] Updated API spec for placeholder users

[Kharonus]

#41299

WAT??

• added inheritance hirarchy for principal resource models
• added placeholder user paths
• revisited other principal specifications

Thoughts and stuff

Hey guys, the PR is rather big, with a lot of text. The reason for that is, that placeholder users were not documented at all, yet. And also affect the documentation about users and groups.

I hope, the changes are understandable, yet, there are some open questions:

• there is a group filter for placeholder users, and apparently I can even add it to a group via API. But I wasn't able to do so via UI. What is our intention here?
• There is an internal status set for placeholder users. Yet, it is never returned by the API, leading to a weird situation, that when you delete placeholder users you still get the data in the collection response. You cannot see a difference, as the status is not rendered.