feat: Implement the new partial mount provider API

[artonge]

To improve the performance of the file system, we introduced a new API to prevent loading all of a user mount points on every request. As Collectives is a mount provider, it would be nice if it implemented the new API. The API consists only in one new method similar to getMountsForUser but limited to a given path.

• Overview of the project: Path specific file sytem setup server#54562
• The new interface: https://github.com/nextcloud/server/blob/master/lib/public/Files/Config/IPartialMountProvider.php
• Implementation in other apps:
  • Files sharing: feat(files_sharing): implement partial mount providers server#57292
  • External storage: feat(external-storage): Implement IPartialMountPoint server#57497
  • Groupfolders: feat(MountProvider): Implement IPartialMountProvider groupfolders#4262