volume: add an "io.systemd.StorageProvider" IPC API that is supposed to be used by vmspawn/nspawn/pid1 to provide storage volumes in a generic fashion (#41776)

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

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,