fix(arte): add required property url

The info dict documentation lists url as a required property:
```
url:            Final video URL.
```
https://github.com/yt-dlp/yt-dlp/blob/master/yt_dlp/extractor/common.py#L281

The extractor should also return this property.

Initially, I thought `webpage_url` would not be in the documentation,
so I renamed `webpage_url` to `url`.
Later, I learnt that there is also the optional property `webpage_url`.
This is why I'm adding the required url and keeping `webpage_url` as is.

yt_dlp/extractor/arte.py

@@ -214,6 +214,7 @@ class ArteTVIE(ArteTVBaseIE):
 
         return {
             'id': metadata['providerId'],
+            'url': traverse_obj(metadata, ('link', 'url')),
             'webpage_url': traverse_obj(metadata, ('link', 'url')),
             'title': traverse_obj(metadata, 'subtitle', 'title'),
             'alt_title': metadata.get('subtitle') and metadata.get('title'),