Add Tileset::sampleHeightCurrentDetail

For my use case, I'd like to have a method Tileset::sampleHeightCurrentDetail which would work similarly to Tileset::sampleHeightMostDetailed except that it operates synchronously and derives the answer from whatever LOD is currently loaded in the tileset rather than progressively loading the tileset down to the finest level of detail. Does that functionality already exist somewhere in Cesium Native? If it doesn't already exist, is that a feature that would be desirable to add? If it would be desirable, would you be interested in a pull request for it? (I'm going to implement it myself regardless, but I'm wondering if it's worth the extra effort to turn it into a proper PR.)

Start by locating the existing Tileset::sampleHeightMostDetailed entry point and its related tests or documentation. Compare its behavior with the requested synchronous operation, then verify that the new method uses whatever LOD is currently loaded without progressively loading finer tiles; done means the API and its behavior are covered by appropriate tests.

Written by the indexing model from the issue text.