[guest users] Invitations API first steps #2320
Description
Create a spec for the guest user invitation API. Create a basic implementation that persists the invitations.
- Align with MSGraph API (https://learn.microsoft.com/en-us/graph/api/resources/invitation?view=graph-rest-1.0) where it makes sense
- Decide for a persistence layer (CS3/NATS/...)
- the implementation does NOT need to send out mails (yet)
Acceptance Criteria
- Specification is added to libregraph-api
- Invitation can be created, listed and revoked