Subtitle Encoder with Transformer Api

[ranjitsingha]

Before filing a feature request:

• Search existing open issues, specifically with the label ‘enhancement’:
  enhancement
• For ExoPlayer-related feature requests, please also check for existing feature
  requests on the ExoPlayer tracker:
  https://github.com/google/ExoPlayer/labels/enhancement
• Search existing pull requests:
  • On this tracker: https://github.com/androidx/media/pulls,
  • On the ExoPlayer tracker: https://github.com/google/ExoPlayer/pulls

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[REQUIRED] Use case description

Use of media3 transformer to encode/hardcode caption/subtitles to a video.

[andrewlewis]

This looks the same as #2333 but as an enhancement request.