[Feature Request] Option to export to any path defined by string rather than documents UI

[LillyWho]

So I've got a bit of an edge case here, I'm trying to push the mp3 files automatically into an nfs share on my raspi server (which cannot be mounted anywhere on the internal /sdcard), but I'm sure there are some other uses for being able to just give a plain filesystem path. Files would probably have to be written as root rather than unprivileged writes.

[jcgruenhage]

While this would be possible (the default currently is built from a String), I'd prefer it if someone wrote a DocumentsProvider for NFS. That solution would then instantly work with all StorageAccessFramework enabled apps (like this one), and require no higher privileges.