Add topSongsByArtistId extension

[sentriz]

getTopSongs currently only accepts an artist name. Which is ambiguous since two
artists can share the same name, so the server has to guess which one the client
meant

Passing an artist ID removes the ambiguity

• A server backed by something like ListenBrainz (MBID based) can resolve the
  exact artist directly, disambiguating same named artists
• Even a string-based backend like last.fm benefits. The server can resolve the
  artist from the ID it already knows, then fetch/filter that artist's top songs
  precisely instead of round-tripping a name

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[Tolriq]

For the record this also fit #258 ;) filter artistId sort by topSongs and is one more argument for it ;)