Add a public api for apps to add mounts

This adds a public api which apps can use to add additional mountpoints in a more structured way then listening to hooks.

First PR for #12216

cc @PVince81 @DeepDiver1975 
Hmm... rename to "getMountProviderCollection()" ? We should probably continue discussing the name.
I'm not super happy with `MountProviderCollection` but can't find anything better just now.