sysupdate: Allow multiple documentation URLs for a feature

sysupdate: Allow multiple documentation URLs for a feature

Change the varlink API for Feature structs to allow multiple
documentation URLs for them, to match what systemd already does for
units etc.

This is a deviation from what the sysupdated D-Bus API allows and, for
the moment, from what’s supported internally by sysupdate. Internally it
continues to support 0-1 URLs for now.

But by defining the API as a strv, multiple URLs can be supported in
future without API breaks.