profiles: Implement vk.xml `feature` xml element

[christophe-lunarg]

        <extension name="VK_KHR_robustness2" number="613" author="KHR" type="device" contact="Piers Daniell @pdaniell-nv" depends="VK_KHR_get_physical_device_properties2,VK_VERSION_1_1" supported="vulkan" ratified="vulkan">
            <require>
                <enum value="1"                                                 name="VK_KHR_ROBUSTNESS_2_SPEC_VERSION"/>
                <enum value="&quot;VK_KHR_robustness2&quot;"                    name="VK_KHR_ROBUSTNESS_2_EXTENSION_NAME"/>
                <enum offset="0" extends="VkStructureType" extnumber="287" name="VK_STRUCTURE_TYPE_PHYSICAL_DEVICE_ROBUSTNESS_2_FEATURES_KHR"/>
                <enum offset="1" extends="VkStructureType" extnumber="287" name="VK_STRUCTURE_TYPE_PHYSICAL_DEVICE_ROBUSTNESS_2_PROPERTIES_KHR"/>
                <type name="VkPhysicalDeviceRobustness2FeaturesKHR"/>
                <type name="VkPhysicalDeviceRobustness2PropertiesKHR"/>
                <feature name="robustBufferAccess2,robustImageAccess2,nullDescriptor" struct="VkPhysicalDeviceRobustness2FeaturesKHR"/>
            </require>
        </extension>