[plugins.json] Add cocoapods-integrate-flutter

[Prateek Grover]

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

{
  "gem": "cocoapods-integrate-flutter",
  "name": "CocoaPods Integrate Flutter",
  "author": "kerry",
  "url": "https://github.com/EXAMPLE/cocoapods-integrate-flutter",
  "description": "A longer description of cocoapods-integrate-flutter."
}