Sharing the model with `rspotify-model`

[marioortizmanero]

I think it would be very beneficial if both rspotify and aspotify could share the same model crate. The clients differ, but the model is generic enough that it can be used by anyone. We have it at the rspotify-model directory. It's hard to keep the model up to date, and after the big refactor in v0.11 I think ours is in a state that's good enough for sharing.

What do you think?