feat(subsonic): fetch artist images from lastfm opengraph

closes #295

Co-authored-by: cacko <alex@cacko.net>

server/ctrlsubsonic/handlers_by_tags.go

@@ -338,6 +338,12 @@ func (c *Controller) ServeGetArtistInfoTwo(r *http.Request) *spec.Response {
 				sub.ArtistInfoTwo.LargeImageURL = image.Text
 			}
 		}
+		if url, _ := lastfm.StealArtistImage(info.URL); url != "" {
+			sub.ArtistInfoTwo.SmallImageURL = url
+			sub.ArtistInfoTwo.MediumImageURL = url
+			sub.ArtistInfoTwo.LargeImageURL = url
+			sub.ArtistInfoTwo.ArtistImageURL = url
+		}
 	}
 
 	count := params.GetOrInt("count", 20)

server/ctrlsubsonic/spec/spec.go

@@ -284,6 +284,7 @@ type ArtistInfo struct {
 	SmallImageURL  string           `xml:"smallImageUrl"           json:"smallImageUrl"`
 	MediumImageURL string           `xml:"mediumImageUrl"          json:"mediumImageUrl"`
 	LargeImageURL  string           `xml:"largeImageUrl"           json:"largeImageUrl"`
+	ArtistImageURL string           `xml:"artistImageUrl"          json:"artistImageUrl"` // not sure where this comes from but other clients seem to expect it
 	SimilarArtist  []*SimilarArtist `xml:"similarArtist,omitempty" json:"similarArtist,omitempty"`
 }