GH-118128
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Validate extension JSON: Error: Field 'classes/OpenXRSpatialAnchorCapability/methods/create_new_anchor/arguments': size changed value in new API, from 2 to 3.
Validate extension JSON: Error: Field 'classes/OpenXRSpatialComponentData/methods/_get_structure_data': is_const changed value in new API, from true to false.

Added optional "next" structure when creating a new anchor.  The "next" structure is passed to the XR runtime.
Made "OpenXRSpatialComponentData::_get_structure_data" (which is the virtual method that GDExtensions can override) non-const, which is consistent with the core virtual "OpenXRSpatialComponentData::get_structure_data" method.
Compatibility methods registered.
