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.
You can also destroy the enemy base with ground troops alone and then go for the SAM sites. I don't think making this a primary objective is really helpful here.