core: introduce io.systemd.Manager.Describe method (#35098)

Let's extend pid1's varlink interface and add a Describe method to get
the global Manager object information as a JSON object
(io.systemd.Manager.Describe).

Because the new varlink interface should be available on both the user
managers and the system manager, we also make the necessary changes to
expose a varlink server on user managers.

This PR is first part of https://github.com/systemd/systemd/pull/33965
with minimal changes to address feedback.

Fixes: #22654