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/libsystemd/sd-varlink/test-varlink-idl.c

@@ -44,6 +44,7 @@
 #include "varlink-io.systemd.Resolve.h"
 #include "varlink-io.systemd.Resolve.Hook.h"
 #include "varlink-io.systemd.Resolve.Monitor.h"
+#include "varlink-io.systemd.StorageProvider.h"
 #include "varlink-io.systemd.Udev.h"
 #include "varlink-io.systemd.Unit.h"
 #include "varlink-io.systemd.UserDatabase.h"
@@ -212,6 +213,7 @@ TEST(parse_format) {
                 &vl_interface_io_systemd_Resolve,
                 &vl_interface_io_systemd_Resolve_Hook,
                 &vl_interface_io_systemd_Resolve_Monitor,
+                &vl_interface_io_systemd_StorageProvider,
                 &vl_interface_io_systemd_Udev,
                 &vl_interface_io_systemd_Unit,
                 &vl_interface_io_systemd_UserDatabase,