Consider using spotify package for web API

[banool]

This library currently builds out its own support for using the web API. There is already a package, spotify, that implements partial support for the web API. If we migrated over to that, we could reduce the amount of work required to maintain this library.

[fotiDim]

@banool it has been discussed and personally I am in favor of it. If someone is up for the task PRs are welcome.

[brim-borium]

@banool Just to clarify, the idea is to use spotify-dart inside the spotify_sdk lib or to merge the two packages and have one package for all?

[banool]

My thoughts were we would do the former, I don't think users of spotify-dart should necessarily take on the dependency of the mobile SDKs if they don't want them.

[brim-borium]

Fine with me :)