Missing domain data classes on just_audio package

[SimoneBressan]

Implement a domain data class on just_audio package for this classes:

• DarwinEqualizerParametersMessage -> DarwinEqualizerParameters
• DarwinEqualizerBandMessage -> DarwinEqualizerBand

NB: Remove a export on just_audio/lib/just_audio.dart file

Personal thinking that requires evaluation:
Consider abstracting the fact that they are platform specific classes. By removing the name and exposing a standard equalizer for both platforms. If unused parameters were passed on one of the two platforms, ignore them and check them with assert