Add ext_video_player for Youtube support

[prateekmedia]

A successor of #505

[diegotori]

Ok, so this might be a bit too restrictive. The reason being is that if the user wants to only use a version of video_player supported by the Flutter Team, this change removes that option.

Ideally speaking, the best approach to support different types of video sources is to create an abstraction over the default VideoPlayerController that ChewieController uses. If you can create that, then it'll allow you to create a new Flutter library that provides a concrete video player controller implementation that uses the VideoPlayerController from ext_video_player.

@brianegan I am curious on your thoughts regarding this, since abstracting away the video player controller might be the way to go in terms of providing multiple video sources outside of video_player's VideoPlayerController.

[prateekmedia]

@diegotori You are right, I will close this as currently I don't have the knowledge to create that.

[diegotori]

@prateekmedia all you would have to do is create a base class that can be used for wrapping the video player controller's functions. Then create a concrete implementation that takes in a VideoPlayerController.

This will definitely be a breaking change, so migration documentation is a MUST for this. It must allow a user to maintain their existing VideoPlayerController by using the appropriate concrete wrapper class with minimal changes. A constructor or factory method that takes in a VideoPlayerController instance will suffice for this.

[prateekmedia]

And will the new implementation needed to be renamed? And if yes what will it be.