vkEnumerateInstanceVersion must have an equivalent Qt API in QVulkanInstance

This must be made usable via QVulkanInstance since one may want to know this in order to figure out what to pass in to QVulkanInstance::setApiVersion() before calling create().

See https://www.khronos.org/registry/vulkan/specs/1.2-extensions/man/html/vkEnumerateInstanceVersion.html

and the versioning notes in https://www.khronos.org/registry/vulkan/specs/1.2-extensions/man/html/VkApplicationInfo.html