sysupdate: Add ListFeatures() and ListTargets() varlink methods (#42900)

Following on from adding the basic varlink scaffolding to sysupdate,
let’s varlinkify a couple of the D-Bus methods. Because varlink doesn’t
have a concept of object paths, the D-Bus path structure which allows a
target to be selected has been squashed down to a target argument for
each relevant method.

Varlinkify the way to list targets, and also the way to list features
because that was simple to do at the same time.

More methods need varlinkifying in the future, but let’s do it in small
and manageable chunks.