Federated plugin refactor

[Jon-Salmon]

This pull request updates the plugin into the federated plugin architecture described here.

I am planning on adding Windows support for this plugin in the future, however, this pull request is a pre-requisite for a few reasons:

• Windows can be implemented without platform channels, so this architecture is required to be able to implement it without adding lots of if(Platform.isWindows) to the base plugins dart files.
• There are a few different ways in which this plugin can be implemented on Windows, each with benefits and drawbacks. This architecture would allow multiple different Windows implementations to be published to pub.dev in the future and the end-user can select an alternative one by just importing it in their projects pubspec.yaml file.

This PR doesn't have any functional changes, but does require the flutter_web_auth_platform_interface plugin to be separately published to pub.dev.