shared: add io.systemd.StorageProvider Varlink interface

shared: add io.systemd.StorageProvider Varlink interface

Generic Varlink API for services that hand out file descriptors to
storage volumes. Three methods: Acquire() returns an fd for a named
volume (optionally creating it from a template), ListVolumes()
enumerates available volumes, ListTemplates() enumerates supported
creation templates. Volume types follow kernel inode-type naming:
blk (block device), reg (regular file), dir (directory).

Intent is that multiple providers can sit behind AF_UNIX sockets in a
well-known directory and be consumed uniformly by nspawn, vmspawn,
the service manager (BindVolume=) and similar tools.