IECoreUSD : UsdGeomSubsets translation support for MeshPrimitive #1777

Job	Run time
build (windows-debug-platform25)	23m 31s
build (macos-arm64-platform25)	12m 52s
build (linux-platform25)	16m 58s
build (linux-debug-platform25)	19m 28s
build (linux-platform24)	13m 45s
build (windows-platform25)	26m 50s
	1h 53m 24s

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