Optionally fetch watch time from YouTube API instead of website

[telnet23]

This PR adds the YOUTUBE_API_KEY configuration option for use with the existing FETCH_YOUTUBE_WATCH_TIME configuration option. If the API key is nonempty, the duration will be fetched from the YouTube API. Otherwise, the duration will be fetched from the YouTube website.

Instructions to create an API key: https://developers.google.com/youtube/registering_an_application

API reference: https://developers.google.com/youtube/v3/docs/videos/list

Fixes #2807

Do you follow the guidelines?

• [x] I have tested my changes
• [x] There are no breaking changes
• [x] I really tested my changes and there is no regression
• [x] Ideally, my commit messages follow the Conventional Commits specification
• [x] I read this document: https://miniflux.app/faq.html#pull-request