[Feature] Add equalizer during playback similar to other platforms

[samiy803]

Amperfly has been excellent but sounds different compared to my Spotify since it's missing EQ. An equalizer would be a very nice feature to have and would be enjoyed by most. Both Spotify and Apple Music implement this.

Seems like it's possible with AVAudioUnitEQ. Let me know if you need help implementing it and I can create a PR.

Thanks!

[BLeeEZ]

This sounds really good. I would love to see an PR for this feature. Just a couple of questions:

1. should there be one equalizer in total or should every device (iPhone, HomePod, Bluetooth headphones, …) have one?
2. Should it only be possible to tune the device (see 1.) or should it be possible to tune every song separately (in addition to the device) too?

[Schlaubischlump]

Related: #137