UsdImagingDataSourceMaterialPrim is missing the the MaterialOverrideSchema

[nicolacandussi]

In Hydra 2.0, in files materialSchema.h and materialOverriedSchema.h it's documented that a material could contain an attribute "materialOverride", allowing to override the material public interface shader parameters.
Unfortunately, UsdImagingDataSourceMaterialPrim, which provides the datasource handle to the material prim data, doesn't look for the presence of the materialOverride data, making it inaccessible to downstream scene indices.

[jesschimein]

Filed as internal issue #USD-10708

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