Implement callback between wrapper and vfs object for hydration requests

Implement callback between wrapper and vfs object for hydration requests

This comes with a test simulating an open request coming from another
process (although in our case it's really just a thread). The actual
hydration works as expected by cfapi, handling of encrypted files is for
now missing.

Signed-off-by: Kevin Ottens <kevin.ottens@nextcloud.com>