Serve Amsterdam ZuidOost assets through an Addressable instead of an AssetBundle

[mvriel]

Prior to this change, the Amsterdam ZuidOost assets -25mb of material- was always loaded at the start of the application. With this change, the intent is for objects to be loaded when needed, instead of on load. This will improve application startup time, base memory usage and clean up the code base.

I have chosen to put the assets into an embedded package (https://docs.unity3d.com/Manual/upm-embed.html) to show how that could work, and to clearly separate the non-core assets.

This PR also introduces the first steps into a Plugin system where

1. In the ConfigLoader scene a PluginManager can load plugins that we locally provide
2. When a Plugin is loaded, a PluginManifest file is retrieved and used to populate the AddLayerPanel / Object Library
   2.a) The PluginManifest is part of the addressable, but because it is labeled as "manifest" it will be put into a separate asset bundle, which is tiny in size
3. When the user opens the Object Library, it will be presented with the option to add a layer that is available in a plugin
4. if the user adds that plugin, it will start to import the full contents of the Adressable and add the layer

Improvements can be a loading screen while the layer is being loaded, this will be in a separate PR.

Because loading assets can now be asynchronous - I have changed the prefablibrary to look and feel as if all actions are asynchronous, even local layers. This will provide a uniform interface instead of trying to remember which was async and which is not.

For the reviewer:

The AddressableAssetsData folder and contents is part of Unity's addressables system, there is not much to review there in terms of contents in this PR.

[mvriel]

I have extracted the Object instantiation / spawning code from the ObjectLibraryButton to be more reusable and have clearer separation of concerns