Add ability to create Cesium clips in our runtimes

[j9liu]

Users may want to clip data from Cesium ion (e.g., key areas of terrain) as glTF, then reintegrate it with terrain or data that is streamed through SaaS. This is possible by importing the model, re-aligning it with the original data, and using CesiumCartographicPolygon to prevent visual overlap.

However, re-positioning the glTF is currently a quite manual process. It would be more intuitive if users could clip from within the editors of our runtimes, as requested on the forum here.

This is just brainstorming, but perhaps this could look like:

• opening a Clips window in the runtime editor
• creating a new Clip, specifying an area with a similar UI to Cesium ion's editor / Cesium Cartographic Polygon
• sending this area as a job to Cesium ion
• importing the glTF from Cesium ion directly, and ideally in the exact same location that it was clipped from

This also might suggest that Clips should store the globe-relative location of the glTF, though I'm not sure how that would work. Perhaps it could be in the glTF extras?