Allow for modifying VTT file for auto-generated captions

[sgrobins]

Currently we are able to create auto-generated captions via the sanity mux plugin.

Feature Request
Add the ability to download the VTT file that contains the captions and then re-upload it with changes.

[bdalziel]

Proposed feature execution:

The ability to replace a language subtitle track allows for editors to fix issues in the auto-generated captions (after downloading and manually editing the text file). The expected flow would be:

1. Provide access to download the full VTT file generated at https://stream.mux.com/{PLAYBACK_ID}/text/{TRACK_ID}.vtt
2. User edits the VTT file using their preferred text editor
3. Within Sanity, editor locates Media corresponding to the video
4. Editor deletes existing track for language (Delete the autogenerated track with the 'delete track' API)
5. Editor uploads VTT file along with language selection (Add a new track to your asset using the edited VTT file, using the create track API)

[ivyrze]

I'm very interested in this as well. I would be happy to submit a PR if folks are open to it.

[ivyrze]

In the meantime until we (hopefully) can integrate this functionality directly, I made a small app called mux-captions to make managing captions a little easier.

[bdalziel]

For reference: the capabilities described in this feature request are now supported in the Contentful / Mux plugin: https://www.mux.com/blog/manage-captions-and-get-production-ready-mux-player-code-in-the-latest-update-to-the-mux-contentful-app