feat: add VTT file support to Document Extractor

[fujita-h]

Summary

VTT files are used as transcription result files in web conferencing systems such as Zoom and Teams. There are cases where these contents are passed to LLM to be summarized. Therefore, we would like to be able to extract VTT files to Document Extractor as plain text files.

Resolves #11147

Checklist

Important

Please review the checklist below before submitting your pull request.

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods