Improve data flow in the async package

[Vasco-jofra]

This PR makes these changes:

1. In FlowSummaryPrivate.qll: Added support for anyProperty content set in flow summaries:
   • Please confirm this is the correct way to support this. I needed this to find the async_.map({a: source()}, call_sink) case. Let me know if there's a better way to do it
   • As a note, I was also unable to use Element, which, according to the documentation, should select "an element of an array, iterator, or set object."
2. In AsyncPackage.qll: Improve taint tracking through functions from the async package:
   • Improve tracking of the callback arguments
   • Implemented flow summaries for more robust taint tracking
   • Updated tests

[Vasco-jofra]

Also, let me know if there's a way to avoid having two DataFlow::SummarizedCallable when all I want to do is change the argument number. I found no way to get a reference to the call in the propagatesFlow predicate against which I could call getIteratorCallbackIndex.