varlink: add new io.systemd.Job interface

This commit creates a new varlink-io.systemd.Job.c file and puts
the job related varlink types into this file. Those will be
used by the upcoming io.systemd.Unit.StartTransient.

Note that the property names follow the D-Bus naming to make a
future "systemctl show" transition from D-Bus -> varlink easier.

Thanks to @ikruglov for suggesting this preparation commit and
to Lennart for suggesting the D-Bus compatibility considerations.

src/shared/meson.build

@@ -223,6 +223,7 @@ shared_sources = files(
         'varlink-io.systemd.Hostname.c',
         'varlink-io.systemd.Import.c',
         'varlink-io.systemd.InstanceMetadata.c',
+        'varlink-io.systemd.Job.c',
         'varlink-io.systemd.Journal.c',
         'varlink-io.systemd.JournalAccess.c',
         'varlink-io.systemd.Login.c',