Move plugins, logic to core

[gkjohnson]

• Move GoogleCloudAuthPlugin, CesiumIonAuthPlugin to core plugins.
• Add override implementations for event listeners.
• Add "assetTypeHandler" to CesiumIonAuthPlugin.

TODO

• CesiumIonAuth: only log error if imported from three
• Add Cesium Ion Plugin to core, add user-defined handler for asset types
• See if "UnloadTilesPlugin" and "UpdateOnChangePlugin" can be moved to core
  • Functions require three.js math primitives
• Consider methods for reorganizing core logic to share more functions (make issue) Migrate more logic to core #1375
  • Move as many events as possible to core
  • See if more update() logic can be moved to core