Optimization of offsets for MAXAR_content_geojson string properties

[timoore]

Description

Offsets to string property values were always stored as 64 bit integers. This PR chooses the appropriate size, which can be UINT8, UINT16, UINT32, or UINT64

Issue number or link

#1414

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.

Remaining Tasks

Testing plan

Run a client with a MAXAR_content_geojson tileset and verify that string metadata is still coherent.

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[j9liu]

Thanks @timoore! I tried this out in Unreal and confirmed that the string data still returns correctly.

Just a few minor comments before merge.

[j9liu]

Update CHANGES.md for this with a note under "Fixes"?

[j9liu]

If this shouldn't happen, perhaps indicate so with a debug / CESIUM_ASSERT statement instead?

[j9liu]

NIT: I was confused by what "refer to" meant on the first read, but maybe this would be clearer?

Suggested change

	// Offsets refer to arrays, so their maximum possible size is the platform's
	// size_t.
	using StringOffset = size_t;
	// Since offsets refer to the offset of array elements, their maximum
	// possible size is the platform's size_t.
	using StringOffset = size_t;