feat: square crop albumart option

[lovegaoshi]

this PR adds an option to crop the album art into a square, so android < 13 and Samsung users will see the squared album art spanning their notif area, instead of a wide one with while spaces on top and bottom.
closes #79
issue is probably better described here (by the flutter colleagues):
ryanheise/audio_service#1035

[dcvz]

Sorry for the late follow-up haere @lovegaoshi. I think overall I'm ok with this, but I'm wondering.. with this we still leave it up to users of the library to figure out whether they need to enable this or not. I wonder if there's a way we can figure this out for them -- perhaps more of a automaticallyDetectArtworkCrop or so..

I wonder what ExoPlayer does in their DefaultMediaDescriptionAdapter.java

[lovegaoshi]

I'm not sure about an automatic way; and since YT also doesnt get this right I doubt this will be automatic. I only know samsung/oneUI all has this square crop need (and I believe their "immersive" album art display introduced with oneUI 6 actually secretly square crops too, but not as the small, actually square thumbnail which is super annoying), and for pixels its weird to crop at all. Not sure/dont care about other OSes.
Actually I just hardcode samsung in my fork
in addition YTM actually adds side filler bars to a perfectly square album art to make it landscape and ugly, if I'm making a YTM clone I'd love to opt in for this, but maybe not for a YT clone

[imagination24]

I need this feature, when will it be merged?