Share user invite between all administrators

[vxgmichel]

Follow up after #9078

Now that the server and data model supports it, this feature can be fully implemented.

Two different approaches have been considered:

1. Add an extra step in the API so that the user can explicitly choose which administrator to contact. This requires to update libparsec, the bindings and the frontend.

2. Update UserClaimInitialCtx::wait_peer to simultaneously wait for all the administrators until one of them joins. This still requires to update libparsec but less changes to bindings and the frontend.

The simpler approach should first be done as a PoC to validate the feature.