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 
Bringing back an older question: should the IMountPoint instance know where it is mounted ? Or should this be stores on a highler level, the mount manager ? But maybe that information is needed in this level already to be able to move mount points.

On the other hand you could say that `FileInfo` represents a file and it also contains the full path, so might be fine.