polkit: turn "interactive" flag to polkit APIs into a proper flags field (#31715)

polkit: turn "interactive" flag to polkit APIs into a proper flags field (#31715)

This adds for both the D-Bus and the Varlink flavours of our polkit
client api a flags parameter. And then folds the "bool interactive" flag
that the D-Bus version so far had, into a flag, and also adds support
for it in the Varlink API.

Since this means the Varlink API gained another parameter, let's do what
we already did for the D-Bus API and add a _full() version of the API
that has the flags and the good_uid parameter, and one without both.