get_tracks adding schemas track.bpm and track.key_signature

[eetuhuhtamaki]

Hello, first of all thank you for this wonderful API. Now that Spotify API deprecated getting tempo and key data for tracks, I would imagine that many people including me will consider using your API for getting this data. Thus, adding track.bpm and track.key_signature would be useful.

[modpotato]

soundcloud doesnt publish bpm and key signature data, it could be implemented if you stream the hls and feed it into a library dedicated to that but thats out of scope for this project iirc