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