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 
Can this be merged into IMountManager or something ?

It seems this class does two things: instantiate the proper storage class (but that can maybe be done in a different way now) and then wrap the storage with wrappers.