Add MTLAccelerationStructureUserIDInstanceDescriptor

[kvark]

Fairly straightforward.

Corresponding piece of "MTLAccelerationStructure.h":

typedef struct {
    /**
     * @brief Transformation matrix describing how to transform the bottom-level acceleration structure.
     */
    MTLPackedFloat4x3 transformationMatrix;

    /**
     * @brief Instance options
     */
    MTLAccelerationStructureInstanceOptions options;

    /**
     * @brief Instance mask used to ignore geometry during ray tracing
     */
    uint32_t mask;

    /**
     * @brief Used to index into intersection function tables
     */
    uint32_t intersectionFunctionTableOffset;

    /**
     * @brief Acceleration structure index to use for this instance
     */
    uint32_t accelerationStructureIndex;

    /**
     * @brief User-assigned instance ID to help identify this instance in an
     * application-defined way
     */
    uint32_t userID;
} MTLAccelerationStructureUserIDInstanceDescriptor API_AVAILABLE(macos(12.0), ios(15.0));