Improve data flow in the `async` package

[Vasco-jofra]

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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