Plugin available here: https://github.com/Jaggernaut555/midi-mixer-spotify/releases
This plugin allows you to control Spotify's volume, mute, play/pause, next track, previous track, and liking a track. It does require some set up as described on the plugin's info page or the github's readme and has some limitations.
This plugin requires a Spotify premium account to work correctly. Spotify limits their API to premium accounts so it's unlikely that there will be a way for this to work without one. This plugin also does not replace running the Spotify app. You will still need to run the application either on your computer or some other device such as a google home.