`GeoJsonDataSource` should have a `process` method similar to CzmlDataSource

The current implementation of `GeoJsonDataSource` does not give any way to dynamically load features in multiple batches, because [it always clears the entity collection](https://github.com/CesiumGS/cesium/blob/1.76/Source/DataSources/GeoJsonDataSource.js#L1042) when `load` is called. `CzmlDataSource` offers a `process` method which simply calls the underlying (private) `load` method without clearing existing entities. This method should be added to `GeoJsonDataSource` for consistency.

(I previously had a comment here about doing the same thing for `KmlDataSource`, but on reflection that doesn't make sense. CZML and GeoJson are based on packets/features and it makes sense to support stream-wise loading; KML is a document format that usually includes e.g. style references and it makes sense to only support loading when the whole thing is available.)

Start in Source/DataSources/GeoJsonDataSource.js around the load implementation at line 1042, then compare it with CzmlDataSource's process method. Define completion as GeoJsonDataSource supporting multiple batches without clearing the existing entity collection, consistently with the stated CZML behavior.

Written by the indexing model from the issue text.