Support reading "Ids" from UsdGeomPointInstancer in shaders

[crydalch]

UsdGeomPointInstancer has an "ids" attribute, which is not available by default to shaders. This necessitates users author primvars:id or primvars:ids separately. It would be great if this were made available to shaders without requiring an additional Primvar authoring, if possible. A long time ago the idea of was discussed on the usd-interest mailing list and validated. However, it turns out I never filed an issue for it - my apologies, making up for lost time now :)

PR addressing this incoming shortly.

Thanks!

[jesschimein]

Filed as internal issue #USD-11871

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