Simplify trigger file for sink argument propagation

[bcaller]

This changes the schema of the trigger file.

Previously there were too many options and it was confusing. My fault,
sorry.

This meant that db.execute(query, **TAINT) was marked as a
vulnerability whereas db.execute(text=query, **TAINT) wasn't.

Neither are vulnerabilities, so this gave a FALSE POSITIVE.

Now we have arg_dict which is a dictionary of keyword to argument
position.

E.g. for def f(a, b, *, c) we can specify the arg_dict as:

{"a": 0, "b": 1, "c": null}

if we want them all to propagate or not propagate depending on the
unlisted_args_propagate value.

This way, we can more easily define db.execute as:

            "execute(": {
                "unlisted_args_propagate": false,
                "arg_dict": {
                    "text": 0
                }
            },

[KevinHock]

LGTM, simplified indeed 👍