Add support for 3DTILES_implicit_tiling_custom_template_variables

[lilleyse]

Description

Adds experimental support for 3DTILES_implicit_tiling_custom_template_variables

This extension allows arbitrary property IDs to be used in implicit tiling template URIs, e.g.

"content": {
  "uri": "content/{level}/{x}/{y}/{timestamp}/{revision}.glb"
}

This is useful for resolving tile content through mechanisms other than just its implicit tile coordinates, e.g. locating content resources by timestamp or revision number.

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

AI acknowledgment

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[github-actions]

Thank you for the pull request, @lilleyse!

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