[FEATURE REQUEST] Transparent Background for Android Icons

### :speech_balloon: Description
On my Android phone, I have a number of apps that have cool icons with transparent backgrounds (such as Paint, Notepad, Music, Camera...). I have been trying to create icons with transparent backgrounds for a while now, but with now success. Obviously, it is technically possible, but it does not seem to be possible with flutter_launcher_icons (or if it is, there's no obvious way to do it, because I tried a whole bunch of things and none worked). I also tried manually editing the android resource files and configurations, but with no success. So, since this is apparently possible but complicated, it would be great to have the functionality in flutter_launcher_icons to automatically do whatever it takes to get it to work.

One thing I tried and which did not work was `adaptive_icon_background: "#00000000"`, so I think maybe something like `adaptive_icon_background: "transparent"` would be a good way to configure this.

### :question: Platform
Android