fixed up the featured artist thing

src/musicbrainz.rs

@@ -139,6 +139,7 @@ impl SearchProvider for MusicBrainzSearch {
                 secondary_types: rg.secondary_types,
                 first_release_date: rg.first_release_date,
                 first_release_id: rg.first_release_mbid,
+                featured: rg.featured,
             })
             .collect())
     }

src/provider.rs

@@ -100,4 +100,6 @@ pub struct ReleaseGroupResult {
     pub secondary_types: Vec<String>,
     pub first_release_date: Option<String>,
     pub first_release_id: Option<String>,
+    /// True if the queried artist is not the primary credit.
+    pub featured: bool,
 }