Parse artist groups (#50)

Co-authored-by: Eugene Simonov <eugene.simonov@evergen.com.au>

database.go

@@ -119,6 +119,7 @@ type Artist struct {
 	ResourceURL    string   `json:"resource_url"`
 	URI            string   `json:"uri"`
 	URLs           []string `json:"urls"`
+	Groups         []Member `json:"groups"`
 	DataQuality    string   `json:"data_quality"`
 }