Multi source terrain provider

[juunie-roh]

Description

This is an initial draft of migration from external library TypeScript implementation into Cesium development environment.

The naming conventions are not considered in this draft.

You can see the detailed description about this feature at the related issue, #12817.

Issue number and link

This PR relates to #12817.

Testing plan

You can see conducted unit tests at external library repository.

Author checklist

• I have submitted a Contributor License Agreement
• I have added my name to CONTRIBUTORS.md
• I have updated CHANGES.md with a short summary of my change
• I have added or updated unit tests to ensure consistent code coverage
• I have updated the inline documentation, and included code examples where relevant
• I have performed a self-review of my code

[github-actions]

Thank you for the pull request, @juunie-roh!

✅ We can confirm we have a CLA on file for you.

[juunie-roh]

There is a limitation in the current implementation:

• Cannot override higher zoom levels within configured terrain regions

For example:

• Level 15 region configured with Provider A
• Level 16+ in the same geographic area should use Provider B
• Currently: Level 16+ continues using Provider A

What causes this is:

• Simple region designation cannot change TerrainAvailability.
• Once a region matches, it returns immediately without checking further options.

This could be solved by nesting HybridTerrainProvider instances, but I'm not sure if it's the right direction.

I haven't deeply investigated this limitation yet, as it may be an edge case.