[video_player] : Add video track selection support for Android and iOS

[nateshmbhat]

This PR adds video track selection (quality switching) support to the video_player plugin for Android and iOS platforms. This allows users to programmatically get available video quality variants and select a specific quality track, similar to the existing audio track selection feature.

Addresses issue : flutter/flutter#58854

Pre-Review Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] page, which explains my responsibilities.
• I read and followed the [relevant style guides] and ran [the auto-formatter].
• I signed the [CLA].
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I [linked to at least one issue that this PR fixes] in the description above.
• I updated pubspec.yaml with an appropriate new version according to the [pub versioning philosophy], or I have commented below to indicate which [version change exemption] this PR falls under[^1].
• I updated CHANGELOG.md to add a description of the change, [following repository CHANGELOG style], or I have commented below to indicate which [CHANGELOG exemption] this PR falls under[^1].
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which [test exemption] this PR falls under[^1].
• All existing and new tests are passing.

[gemini-code-assist]

Code Review

This pull request introduces video track (quality) selection support for Android and iOS in the video_player plugin. It adds getVideoTracks, selectVideoTrack, and isVideoTrackSupportAvailable methods to the VideoPlayerController and the platform interface. The Android implementation leverages ExoPlayer's track selection, including a workaround for resolution changes, while the iOS implementation uses AVAssetVariant for iOS 15+ and preferredPeakBitRate. The web platform includes stub implementations since the feature is not supported there. The changes are well-structured, with corresponding updates to the example app and comprehensive tests. I have one suggestion to improve the example app's robustness by avoiding the use of BuildContext across asynchronous gaps.

[gemini-code-assist]

Avoid using BuildContext across asynchronous gaps. The context might be invalid after an await. It's safer to get a reference to ScaffoldMessenger before the async calls.

  Future<void> _selectVideoTrack(VideoTrack? track) async {
    final VideoPlayerController? controller = _controller;
    if (controller == null) {
      return;
    }
    final ScaffoldMessengerState scaffoldMessenger = ScaffoldMessenger.of(context);

    try {
      await controller.selectVideoTrack(track);

      setState(() {
        _isAutoQuality = track == null;
      });

      // Reload tracks to update selection status
      await _loadVideoTracks();

      if (!mounted) {
        return;
      }
      final String message = track == null
          ? 'Switched to automatic quality'
          : 'Selected video track: ${_getTrackLabel(track)}';
      scaffoldMessenger.showSnackBar(SnackBar(content: Text(message)));
    } catch (e) {
      if (!mounted) {
        return;
      }
      scaffoldMessenger
          .showSnackBar(SnackBar(content: Text('Failed to select video track: $e')));
    }
  }