feat: support adjust padding bottom for branding image

[Lzyct]

What it does

• Support adding bottom padding for branding image regarding this issue https://github.com/jonbhanson/flutter_native_splash/issues/694

How to test

1. follow this documentation to run and generate the native splash screen
2. enable branding image

3. add this config on your config yaml

   branding_bottom_padding: 24
   
   #branding_bottom_padding_ios: 24 #specific config for iOS
   #branding_bottom_padding_android: 24 #specific config for Android

4. observe the branding image position

Screenshot

BEFORE

iPhone 15 Pro Max

iPhone SE

< Android 13

AFTER

iPhone 15 Pro Max

iPhone SE

< Android 13