[video_player_avplay] Add subtitle attributes which include type,startTime, stopTime and value to subtitle callback

[gin7773]

Main changes:

• Upgrade native player.
  1. [PLAYER]Add definitions of the subtitle attributes.
  2. [PLAYER]Receive and pass subtitle attributes of streaming to video player in subtitle callback.
• Handle the subtitle attributes which include type, startTime, stopTime and value.
• Pass subtitle attributes to VideoPlayerController.

[JSUYA]

If you have time, it would be good to apply this feature to the main.dart of the example to print the attribute of the subtitle. We are adding many interfaces of avplay, but there is not enough page or example to guide(and test) all of them.

[gin7773]

If you have time, it would be good to apply this feature to the main.dart of the example to print the attribute of the subtitle. We are adding many interfaces of avplay, but there is not enough page or example to guide(and test) all of them.

According to the requirement of the third-party, the TextStyle needs to be processed according to the subtitle attribute. So I will apply this feature to the main.dart after completing this requirement.