Add Tileset::sampleHeightCurrentDetail() to sample terrain height based on the currently loaded tiles

[ELeeScape]

Description

Tileset::sampleHeightMostDetailed() run asynchronously and causes the tileset to fully load the most detailed level before resolving a result. In some cases it's more useful to know what the height of the currently-loaded LOD is, and to know it immediately.

Refactored a bit of logic out of TilesetHeightRequest to TilesetHeightQuery in order to reduce code duplication with the new functionality.

Issue number or link

Fixes #1216.

Author checklist

• I have submitted a Contributor License Agreement (only needed once).
• I have done a full self-review of my code.
• I have updated CHANGES.md with a short summary of my change (for user-facing changes).
• I have added or updated unit tests to ensure consistent code coverage as necessary.
• I have updated the documentation as necessary.

Testing plan

Added unit tests to cover additive-refined, replace-refined, and partially-loaded tilesets as well as exercising the convenience overload.

Reviewer checklist

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• Review and run all parts of the test plan on this branch and verify it matches expectations.
  • If the issue is a bug please make sure you can reproduce the bug in the main branch and then checkout this branch to make sure it actually solved the issue.
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  • Code should follow the C++ Style Guide
• Review the unit tests and make sure there are no missing tests or edge cases.
• Review documentation changes and updates to CHANGES.md to make sure they accurately cover the work in this PR.
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[ELeeScape]

I'm not deeply familiar with cesium-native internals yet so I'm happy to receive feedback on better ways to implement this!