core: introduce varlink `io.systemd.Job` interface (#42104)

Methods:

- `io.systemd.Job.List` — list all queued jobs or look up by `id`/`unit`
name, with streaming support. Uses context/runtime split: `JobContext`
(Unit, JobType) and `JobRuntime` (Id, State, Result,
ActivationDetails). Follows the same SELinux and parameter-conflict
patterns as `io.systemd.Unit.List`.
- `io.systemd.Job.Cancel` — cancel a specific job by ID, with SELinux
and polkit authorization.
- `io.systemd.Job.ClearAll` — cancel all pending jobs, with SELinux and
polkit authorization.

diff --cc src/core/meson.build
Simple merge

diff --cc src/core/varlink-unit.c
Simple merge

diff --cc src/core/varlink.c
Simple merge

diff --cc src/libsystemd/sd-json/test-json.c
Simple merge