Ported mission GDI03 to the new Lua API

Followup of https://github.com/OpenRA/OpenRA/pull/6786, https://github.com/OpenRA/OpenRA/pull/6769 and https://github.com/OpenRA/OpenRA/pull/6762.
It sounds like our crushing / pathing logic needs improving to account for cases where units can explicitly (force-move) crush actors, but shouldn't by default.