New user-management API changes

[bastihav]

Description

Implements upb-uc4/api#74

Type of change (remove all that don't apply)

• Breaking change (fix or feature that would cause existing functionality to not work as expected)

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Backend: (remove all that don't apply)
  • deployed experimental build

Checklist: (remove all that don't apply)

• I added corresponding unit tests (if API code)
• I have performed a self-review of my own code
• My changes generate no new linting warnings or console warnings
• I have updated the CHANGELOG.md to include any changes made in this PR (add WIP to the top, if there is none already)

[tobias-wiese]

<3

(As it's not deployed yet, I obviously could not test it, but it looks fine)