feat: SCIM support

[sebadob]

I had no real use for SCIM so far and nobody wanted to provide a PR for it, which is why the support is not there yet.
But, I do need it now, at least the most important operations like user CRUD for instance.

After the UI Migration, User Avatars and Backchannel Logout are done, I will add the most important SCIM functionality.

• UI Migration finished
• User Avatars finished
• Backchannel Logout finished

If someone else has an interest in having SCIM earlier, feel free to submit a PR.