Flutter permission_handler plugin

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

1. permission_handler: 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 permission_handler plugin you can refer to its README.md file. At this moment the Android and iOS platform implementations are also part of this package. Additional platform support will be added in their own individual "platform package(s)".
2. permission_handler_platform_interface: this packages declares the interface which all platform packages must implement to support the app-facing package. Instructions on how to implement a platform packages can be found in the README.md of the permission_handler_platform_interface package.