Fix heatsink pads in generated ExposedPads (heatsink property and zone connections)

This commit series adds (type-safe) fabrication properties to the Pad class, and appropriate serialisation to s-expr.

This is then used to:

• Address kicad-footprints#2634 - add 'heatsink' property to ExposedPad , which will be used by _SO and QFN generators
• Address kicad-footprints#2564 (closed) - add 'bga' property to BGA pads in the BGA generator

Then, add zone connection settings to Pad, and serialisation. Use this for:

• Making EPs have solid zone connections, which is more likely to be true on a thermal pad

This doesn't resolve the issue, as regeneration is also required, which will be a follow-up.

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For heatsink pads, the property is applied to all pads: the two copper pads (front and back, if the back pad exists) and all thermal "vias" (which are also Pads)

Attached some examples:

• QFN with 'heatsink' pads:

  Texas_S-PVQFN-N48_EP5.15x5.15mm_ThermalVias.kicad_mod

• BGA with 'bga' pads:

  Alliance_TFBGA-54_8x8mm_Layout9x9_P0.8mm.kicad_mod

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Suggested KLC update: klc!48 (closed)