Allow to use a font asset on Android

[Abestanis]

This adds an optional parameter fontAsset to the showToast method that allows to apply a custom font from the Flutter app assets folder to the text of the toast on Android below API level 30 (Android R) where we have access to the TextView. Credit for the change goes to @nt4f04uNd. We have been using a fork with these changes for the past year.

Usage example:

Fluttertoast.showToast(
  msg: msg,
  fontAsset: 'assets/path/to/some-font.ttf',
);

Normal toast	Toast with comic sans font
Fluttertoast.showToast(msg: msg);	Fluttertoast.showToast(msg: msg, fontAsset: 'assets/fonts/comic-sans-ms.ttf');

[Abestanis]

Thanks for the merge and fast release! ❤️

[nt4f04uNd]

Thanks for the merge and fast release! ❤️

Incredible, congrats! 🎉

I will be honest, I was surprised with this contribution, because I completely forgot I even did this fork.

Thank you for moving this patch to the package itself. These types of last miles are one of my favorite types of contributions I imagine trying to do at some point myself, but rarely have enough will power.