core: add io.systemd.Unit.StartTransient() to the varlink API (#41583)

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 structured json object "UnitContext" with a "Service" field
inside.
   It is also only implementing a minimal set of what can be done with a
   service.
2. No aux units (for now)
3. When called with --more the varlink socket can notify about
   state changes depending on the notify{Job,Unit}Changes parameter

This aligns to the json objects/format from
https://github.com/systemd/systemd/pull/39391
and to show how the format can be shared it adds a new
(minimal) `ServiceContext` that is now part of
`io.systemd.Unit.List()`.