Migrate Navidrome support to generic subsonic clone support (#55)

Renaming BONOB_* env vars to BNB_*

tests/in_memory_music_service.ts

@@ -125,7 +125,7 @@ export class InMemoryMusicService implements MusicService {
         ),
       stream: (_: { trackId: string; range: string | undefined }) =>
         Promise.reject("unsupported operation"),
-      coverArt: (id: string, _: "album" | "artist", size?: number) =>
+      coverArt: (id: string, size?: number) =>
         Promise.reject(`Cannot retrieve coverArt for ${id}, size ${size}`),
       scrobble: async (_: string) => {
         return Promise.resolve(true);