plugins.ard_mediathek: update to support site change
plugins.ard_mediathek: update to support site change

The "smil" stream endpoint was changed in someway, probably "akamai" was
removed from the stream URL, causing the HDS streams to not be
recognised as akamai streams. This patch checks the cdn value returned
in the smil stream info and sets the akamai flag for HDSStreams
appropriately.

bastimeyer/streamlink,melmorabity/streamlink,mmetak/streamlink,bastimeyer/streamlink,streamlink/streamlink,back-to/streamlink,wlerin/streamlink,melmorabity/streamlink,streamlink/streamlink,javiercantero/streamlink,beardypig/streamlink,chhe/streamlink,gravyboat/streamlink,gravyboat/streamlink,chhe/streamlink,wlerin/streamlink,javiercantero/streamlink,mmetak/streamlink,back-to/streamlink,beardypig/streamlink