support arbitrary creation of resources in the Asset Catalog

A handful of plugins* recommend the use of an image/asset out of the Asset Catalog. It would be great if we could create those just by setting tags in config.xml, similar to <icon> and <splash> that then generate the resource in Xcode.

My specific use case is to add a "back" image to be used by 'cordova-plugin-themeablebrowser'. I can do this manually by:

• opening xCode
• Click on Project -> Resources -> Image.xcassets
• Click '+' in the bottom task bar -> New Image Set
• Rename the new Set 'back'
• Manually drag and drop 3 images into xcode

To be clear - I'm not asking that cordova produce different sized images as I believe that's out of scope. I just need it to build the new resource in xcode referencing certain files on disk with tags like:

<platform name='ios'>
  <resource name='back' src='my_resources/back@1x.png' target='1x' />
  <resource name='back' src='my_resources/back@2x.png' target='2x' />
  <resource name='back' src='my_resources/back@3x.png' target='3x' />
</platform>

• Example plugins:
  https://github.com/arnesson/cordova-plugin-firebase
  https://github.codifferent m/phonegap/phonegap-plugin-push
  https://github.com/initialxy/cordova-plugin-themeablebrowser

*Bonus!*
It'd be extra great, since I'm sure some folks want this, if cordova could add sound files to be used by firebase/push plugins that can leverage those if they exist, but I'm not sure how those get set up in xcode such that phonegap-plugin-push can use them.