`setProperty()` behavior for 3D Tiles Next

The current implementation of 3D Tiles Next has some limited support for `setProperty()` to change property values at runtime in a similar way to how the older batch table worked. Any changes are non-persistent and only exist for a single run.

However, there are still some outstanding questions about the desired behavior of this function, particularly when assigning to a property that doesn't exist (currently disallowed)

* If you `setProperty('unknownProperty', value)` should this add a new field to the current metadata class?
* Should it continue to do nothing?
* Should it add the field to a new class for properties not in the schema?
* Something else?
* If a tileset has no `3DTILES_metadata` extension, should `setProperty()` throw an exception, should it attempt to create an empty schema and add a property to it?

Start by reading the current 3D Tiles Next setProperty() implementation and its handling of properties outside the schema. Resolve the behavior for unknown properties and tilesets without the 3DTILES_metadata extension, then document the decision and verify the resulting runtime, non-persistent behavior.

Written by the indexing model from the issue text.