Add GetPhysicalDeviceExtensionInfo() function to API

Added function to driver, currently returning a count of 0 and VK_ERROR_INVALID_VALUE if an attempt it made to retrieve extension properties.
This change is slightly leading another so the first paramter is currently "VkPhysicalGpu" but should be migrated to "VkPhysicalDevice" when that change comes in.
diff --git a/icd/intel/gpu.h b/icd/intel/gpu.h
index a113a5e..ba32f3b 100644
--- a/icd/intel/gpu.h
+++ b/icd/intel/gpu.h
@@ -41,6 +41,14 @@
     INTEL_EXT_INVALID = INTEL_EXT_COUNT,
 };
 
+/*
+ * No device-specific extensions at thie point, so this enum is a placeholder
+ */
+enum intel_physical_device_ext_type {
+    INTEL_PHY_DEV_EXT_COUNT,
+    INTEL_PHY_DEV_EXT_INVALID = INTEL_PHY_DEV_EXT_COUNT,
+};
+
 enum intel_gpu_engine_type {
     /* TODO BLT support */
     INTEL_GPU_ENGINE_3D,