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.

src/shared/meson.build

@@ -245,6 +245,7 @@ shared_sources = files(
         'varlink-io.systemd.Resolve.Hook.c',
         'varlink-io.systemd.Resolve.Monitor.c',
         'varlink-io.systemd.Shutdown.c',
+        'varlink-io.systemd.StorageProvider.c',
         'varlink-io.systemd.Udev.c',
         'varlink-io.systemd.Unit.c',
         'varlink-io.systemd.UserDatabase.c',