Support VTT transcript file format in addition to SRT

Is your feature request related to a problem? Please describe

VTT is a more flexible and advanced "subtitle" or captioning format that is very similar to SRT, but provides a wider range of formatting options. Podcast players that support VTT may also support these formats, and now Apple has announced transcript support, they will automatically create transcripts in VTT format that can be downloaded and referenced back into the item data. We need to be able to upload a transcript file in VTT format to make this easy.

Describe the solution you'd like

Support uploading VTT transcript format in addition to the SRT format.

Describe alternatives you've considered

• Externally transcoding SRT to VTT - there are many converters out there.
• Manually editing the VTT file to make it SRT (replace periods with commas, remove VTT metadata at head of file etc).

Additional context

Apple reference: https://podcasters.apple.com/support/5316-transcripts-on-apple-podcasts