Unifying Android and iOS plugins into a single easy to use plugin

This change creates a new parallel 'unified' folder that contains the Android and iOS plugins merged together. On the Unity side, we use a single GoogleMobileAdsPlugin interface that calls down to the native APIs depending on which build platform is active in Unity.

The GoogleMobileAdsDemoScript was updated to be more "Unity-like", by providing various editor-facing properties that the user can modify to experiment with the API. It also provides a friendly warning and doesn't call the native APIs (which would result in various errors) when running in the Editor.

The iOS version of the plugin was updated to support the other two pull requests I submitted: [optional 'extras' parameter to RequestBannerAd](https://github.com/googleads/googleads-mobile-plugins/pull/4), and [support for bottom banners](https://github.com/googleads/googleads-mobile-plugins/pull/5). You can easily reverse those changes if you desire.

I also provided a README.md that states this contribution is covered under the Google Individual Contributor License Agreement, and combines the instructions for both the Android and iOS plugins. Feel free to modify/update as needed.

GoogleMobileAdsPlugin.unitypackage was re-exported to contain all of these changes.