feat: honor footprint attributes of current variant

[piernov]

Add support for variants introduced in KiCad 10.

Not thoroughly tested in KiCad 10, and not tested at all in KiCad <= 9.

[silbe]

(I'm not a maintainer, just another user)

Haven't checked each change individually against the API docs but the changes look sensible and complete. The one thing I'm not sure about (and cannot currently try out) is exactly how KiCad v10 represents footprints that are variants of the same symbol and whether that's handled correctly in the plugin. I'm guessing they are represented as independent footprints with the same reference designator and that it triggers the "unique designator" logic. In that case it may be useful to ignore footprints that are going to be excluded while counting the footprint designators in generate_tables() (otherwise the designators in the production files will be different from the ones in the KiCad files even though for each variant the designator is unique).

Tested successfully with v9: When used from a) within the KiCad v9 PCB editor and b) on the command line, both times on a moderately complex design (hierarchical design with several instances of the same sheet) it produces unchanged output (except for the timestamps included in the files) and I don't see any error message or warning on the command line.