[plugins.json] Add cocoapods-embed-flutter

[Soumya Ranjan Mahunt]

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

{
  "gem": "cocoapods-embed-flutter",
  "name": "CocoaPods Embed Flutter",
  "author": "Soumya Ranjan Mahunt",
  "url": "https://github.com/DartBuild/cocoapods-embed-flutter",
  "description": "Straight forward way of declaring flutter modules as dependency for targets, just like cocoapods does with pods."
}

[Soumya Ranjan Mahunt]

Available with #141.