[Feature] New extension - YouTube Duration

### Is your feature request related to a problem? Please describe.

I came up with a new extension to check the lengths of YouTube videos on YouTube channel feeds. The extension then appends the length of the video to the title of the feed item. Here it is:
https://github.com/cevvalkoala/xExtension-YouTubeDuration

### Use case

_No response_

### Describe the solution you’d like

I'd appreciate if you could add it to the list of unofficial extensions.
Thank you.

### Describe alternatives you’ve considered

_No response_

### Additional context

_No response_

Locate the list of unofficial extensions in the FreshRSS/Extensions repository and review the linked YouTube Duration extension. Add the extension to that list with the relevant link, then verify that the updated list is accurate and renders correctly.

Written by the indexing model from the issue text.