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 
Was doubting this myself, I dont think exposing it in the container has any other use then for constructing mount providers and always adding such single use things to the container increases it's size needlesly.

That said, passing it as an argument is not ideal