[plugins.json] Add cocoapods-assets-cleaner

[Leo Valentim]

Please add the following entry to the plugins.json file:

{
  "gem": "cocoapods-assets-cleaner",
  "name": "CocoaPods Assets Cleaner",
  "author": "Leo Valentim",
  "url": "https://github.com/LeoValentim/cocoapods-assets-cleaner/",
  "description": "Cocoapods-plugin that helps to clean unused assets on Xcode projects."
}