Add options to convert unsupported primitive modes when reading glTFs

[j9liu]

Description

Depends on #1388, so merge that first.

This PR supports (but does not complete) #1321.

A new struct, CesiumGltfReader::MeshPrimitiveModeOptions allows clients to request conversions of LINE_LOOP, LINE_STRIP, TRIANGLE_STRIP, and TRIANGLE_FAN, as they are less likely to be supported.

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

• Use cesium-native to retopologize unsupported primitive modes cesium-unreal#1854
• Support rendering for all glTF primitive modes cesium-unity#702

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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• 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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