core: add io.systemd.Unit.StartTransient() to the varlink API

core: add io.systemd.Unit.StartTransient() to the varlink API

This commit adds a simple version of io.systemd.Unit.StartTransient
for varlink. It is similar to the dbus version, but there is a key
difference:
1. Instead of building the unit from key/value properties it
   takes a json object in the "service" parameter. It is also
   only implementing a minimal set of what can be done with a
   service (for now)
2. No aux units (for now)
3. When called with --more the varlink socket can notify about
   unit job and state changes controlled via a bool on the
   varlink call inputs: notify{Job,Unit}Changes

We use the new io.systemd.Job interface when outputing the
io.systemd.Unit.StartTransient result as it makes the output
nice and mirrors the input.

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

Because UnitContext is now also used for the inputs we need
to make a bunch of fields `SD_VARLINK_NULLABLE` so that the
input is even accepted. This does not affect the output, it
is still fully populated, just the schema. The ID of UnitContext
is still required.

Thanks to ikruglov and Lennart for their excellent feedback on
this.