Add track element for transcript/caption file

For various reasons including search engine optimization, it would be helpful if a `track` [element](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/track) was created along with the `video` element in the cloudflare-stream. The track element should reference the WebVTT file uploaded to Cloudflare as captions for the video, if there is one. If I am opening this issue in the wrong repo, please redirect me. Thanks much.

Start at the cloudflare-stream entry point that creates the video element and review how an uploaded WebVTT caption file is exposed. The work is complete when a track element is created alongside the video only when a caption file exists and references that file; the issue names no files or tests to run.

Written by the indexing model from the issue text.