Added Media.PlayMusic to the Lua API

and make use of it on the shellmaps to fix https://github.com/OpenRA/OpenRA/issues/6071.
```
08:10 <+pchote> antares79: does that objective in GDI01 actually do anything?
08:11 <+antares79> I guess it teaches you how to deploy your mcv and how to 
                   build buildings
08:11 <+pchote> I mean in terms of rewards
08:11 <+antares79> no
08:11 <+pchote> that's the real problem
08:12 <+antares79> yep
08:12 <+pchote> if we gated the second set of reinforcements to that, then it 
                wouldn't matter if you then sold/lost a structure
08:12 <+pchote> because you would have already completed it and recieved the 
                reward
08:12 <+pchote> it can stay completed
08:12 <+antares79> agreed
```