Flutter Google API Availability plugin

The Flutter Google API Availability plugin is built following the federated plugin architecture. A detailed explanation of the federated plugin concept can be found in the Flutter documentation. This means the Google API Availability plugin is separated into the following packages:

1. google_api_availability: the app facing package. This is the package users depend on to use the plugin in their project. For details on how to use the google_api_availability plugin you can refer to its README.md file.
2. google_api_availability_platform_interface: this package declares the interface which all platform packages must implement to support the app-facing package. Instructions on how to implement a platform package can be found in the README.md of the google_api_availability_platform_interface package.