[extractors] Use new framework for existing embeds (#4307)

`Brightcove` is difficult to migrate because it's subclasses may depend
on the signature of the current functions. So it is left as-is for now

Note: Tests have not been migrated

yt_dlp/extractor/udn.py

@@ -13,6 +13,7 @@ class UDNEmbedIE(InfoExtractor):
     IE_DESC = '聯合影音'
     _PROTOCOL_RELATIVE_VALID_URL = r'//video\.udn\.com/(?:embed|play)/news/(?P<id>\d+)'
     _VALID_URL = r'https?:' + _PROTOCOL_RELATIVE_VALID_URL
+    _EMBED_REGEX = [r'<iframe[^>]+src="(?:https?:)?(?P<url>%s)"' % _PROTOCOL_RELATIVE_VALID_URL]
     _TESTS = [{
         'url': 'http://video.udn.com/embed/news/300040',
         'info_dict': {