feat: add ios background content mode

[strysk]

Currently, iOS Content Mode for LaunchBackground is always set to "Scale To Fill".
I think it would be nice to be able to set it to other content mode, so added "ios_background_content_mode" parameter.

[jonbhanson]

Thanks for the contribution, but I don't want to expand the background_image parameter. It generally causes confusion when people use it instead of using the image parameter. If anything, I would prefer PR's that resolve this confusion, and limit errors that happen such as when people use ios_content_mode without an image parameter, resulting in a black screen such as #775.

[strysk]

Thanks for your review!
I understand you prefer fixing errors of "ios_content_mode".

However, even if those errors are fixed, there is still no way to change content mode for background image?
That option will be useful for when you use same background image for iPhone and iPad, and you don't wanna change aspect ratio for the image to fit screens.

I found out this package has same option, and it would be nice for flutter_native_splash to have same useful option.
https://pub.dev/packages/splash_master

I would appreciate it if you could reconsider.

[peter100u]

和android端一样，android端同样缺少content_mode