Implement getExtendedMetadata

[switz]

Adding songs to Sonos Favorites errors out with "Something Went Wrong" due to the following Relisten partner bug:

Relisten returns getExtendedMetadata responses for tracks with incorrect format. When requesting getExtendedMetadata for a valid track id, "Track:sci:2007:2007-08-05:1914992:12292369" for example, MSP returns the following:

{
	"getExtendedMetadataResult":{
		"id":"Track:sci:2007:2007-08-05:1914992:12292369",
		"type":"artist"
	}
} 

Notice type = 'artist', not track, and the response object is missing "mediaMetadata" or "mediaCollection" which is a required property on a getExtendedMetadataResult object (see https://docs.sonos.com/docs/getextendedmetadata#tag/playback/operation/Playback-LoadContent-GroupId:~:text=Your%20service%20should,of%20these%20elements )