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.
This is more to alert the player that something happened. Starting with no units is somehow confusing at first. Initial reinforcements are also reinforcements to me.