GTM Custom Function Tags

#256 Support of custom JS function tags in Google Tag Manager, which are triggered from rules setup in your container.

Thanks to @farzadshafiee for the implementation!

Register a function tag by calling:

GoogleTagManager.registerFunctionCallTagHandler(
  "some_function",
  (functionName, tagArguments) => {
    console.log("Handling Function Call tag:", functionName);
    console.log("args:", tagArguments);
 }