Add GeoJson clustering

[glughi]

What this PR does

This PR adds the trait ClusteringTraits to GeoJsonTraits. In this way, it becomes possible to enable the clustering of entities made available by Cesium. Clusterized entities are represented as billboards generated by the fromText function of PinBuilder and report the number of grouped entities. As a result, clustering only works by using Cesium as a viewer, but there are no drawbacks to switching from Cesium to Leaflet at runtime.

Test me

Add a GeoJson or WFS layer of points to the catalog and use new parameters
In example:

"clustering": {
  "enabled": true
},

Checklist

• There are unit tests to verify my changes are correct or unit tests aren't applicable (if so, write quick reason why unit tests don't exist)
• I've updated relevant documentation in doc/.
• I've updated CHANGES.md with what I changed.
• I've provided instructions in the PR description on how to test this PR.