Add api for authoritative mount providers and implement it for files_external

[icewind1991]

• Add the api surface for mount providers to pro-actively update the list of cached mounts
• Implement this for files_external as POC since it's one of the easiest ones

TODO:

• [x] deal with new users, group changes, etc
• [ ] don't get the root cache entry multiple times if there is no user-specific config

Best used with https://github.com/nextcloud/server/pull/56447 to easily see the mount listing