Unify the concept of human being throughout Nextcloud

This is a bit abstract, I know. Bear with me 🙏

The problem

We currently have different categories of "humans" in Nextcloud:

• User (a human behind a Nextcloud account, created by an admin)
• Guest (a human behind a Nextcloud guest account, created by a user)
• Contact (a human that lives in my contact server, not necessarily tied to a Nextcloud account)
• Email user (a human that has been added via email to conversations and files shares)
• Anonymus guest (a human that has joined a conversation via link)
• Federated user (a human behind a Nextcloud account on a different instance)

It would be great if we had:

• a unified concept of human (placeholder term) and work with features and permission of this human to determine its type
• A unified place from where to create and edit humans
• Automatic creation of a new human on the instance if I add an email address to a conversation or to a file
• One click upgradeability of a human: non-user(contact) → guest user → instance user → instance admin

Thoughts @jancborchardt ?

No files, tests, or entry points are named. Start by reviewing how users, guests, contacts, email users, anonymous guests, and federated users are currently represented across Nextcloud. Done requires an agreed unified human concept, creation and editing path, automatic email-based creation, and upgradeability between the listed roles.