C#: Add MaD support for Attribute.Getter and Attribute.Setter.

[michaelnebel]

In this PR we added support for Attribute.Getter and Attribute.Setter as a part of the models as data description in the ext field of a model.
Assume we have the following declarations.

namespace N {
    public class SourceAttribute : System.Attribute { }
  
    public class MyClass {
        [SourceAttribute]
        public object TaggedSrcPropertyGetter { get; }
    }
}

If we add the following source model

- ["N", "SourceAttribute", false, "", "", "Attribute.Getter", "ReturnValue", "local", "manual"]

Then TaggedSrcPropertyGetter is considered a local source of taint.

[michaelnebel]

DCA looks good!

[tamasvajk]

I think we should mention these new extensions in the comment at the top of the file.

[michaelnebel]

Makes sense!

[tamasvajk]

Would we be backwards compatible if we removed these two lines?

[michaelnebel]

Yes, but I think we should take a conscious decision and break the backwards compatibility - as it was quite surprising that tagging a property meant that it worked for setters.
Also, the public documentation doesn't contain the explanation on, how to use the ext field, so it is most likely not heavily used (yet).

[tamasvajk]

LGTM