Play files stored in flutter assets

[andreasgangso]

This is currently possible, at least on android, but very hard to use.
You need to start your url with asset:///flutter_assets/, so e.g. for "assets/demo.mp4", it would look like this:

  MediaItem(
    url: 'asset:///flutter_assets/assets/demo.mp4',
    mimeType: 'video/mp4',
    metadata: MediaMetadata(title: 'Asset file'),
  ),

Instead, we should implement something similar to video_player:

1. add a way to signal that this is an asset file, e.g. add a UrlType in the MediaItem pigeon.
2. on the native side, use something like (untested):

   // Android
   val path = FlutterInjector.instance().flutterLoader().getLookupKeyForAsset(mediaItem.url)
   val exoMediaItemUrl = "asset:///" + path;

   // iOS
   let assetKey = registrar?.lookupKey(forAsset: mediaItem.url)
   let path = Bundle.main.path(forResource: assetKey, ofType: nil)!