[ard] Add support for Beta ARD Mediathek

Thanks to https://blog.fefe.de/?ts=a577685d for pointing out support is missing.

youtube_dl/extractor/extractors.py

@@ -54,6 +54,7 @@ from .appletrailers import (
 from .archiveorg import ArchiveOrgIE
 from .arkena import ArkenaIE
 from .ard import (
+    ARDBetaMediathekIE,
     ARDIE,
     ARDMediathekIE,
 )