volume: add an "io.systemd.StorageProvider" IPC API that is supposed to be used by...

BindPath= in unit files, and --bind= in nspawn/vmspawn doesn't really
cut it to connect arbitrary storage infra to it. Let's do something
about it, and implement a simple, light-weight API for acquiring an fd
to a storage volume. Benefits:

1. the interface can be implemented by anyone, connecting anything to
vmspawn/nspawn/service management
2. very lose coupling: just bind a socket into a well-known dir, done
3. mounting can happen on-demand