Enable Cesium to load and display private datasets

[robyngit]

Currently must be publicly available on the web in order for it to be displayed in Cesium (or hosted through a Cesium ion account, since the api key can be included in the map config). Some groups need to display geospatial data that is private within private portals. For example, a dataset that's not yet released that's only visible to a select group of editors.

Requests for data go through Cesium, so we should first check if there's a way to include headers along with those requests naively in Cesium. If not, then we may need to add a step to fetch the data before passing it to Cesium.

We need to also consider how to fetch images that are private and displayed in the feature info panel (like the thumbnails shown in the PDG portal LEO network layer).

When a user does not have access to data, MetacatUI should handle this gracefully, and suggest they log in if not already, or show a user-friendly message explaining that the data is private.