Add support for aggregate targets

[fortmarek]

XcodeGraph currently doesn't support representing aggregate targets. That means that we have to:

• Skip mapping them from XcodeProj, see fix: skip mapping aggregate targets #140
• We can't add support for these in the project generation: Aggregate Targets (and schemes) tuist#3388

Since aggregate targets don't have all the properties as "regular" targets do, we might need to create a new struct such as AggregateTarget.