Pipeline create info structs can appear in the VkPipelineCreatInfoKHR
pNext chain even though they don't extend that structure in the
traditional sense.
Adding Tracy in VVL showed that we are paying the price of initializing
global variables upon shared library entry, even if we do not
end up accessing those.
Adding accessors will make sure we pay this price
only when truly needed.
Also making the necessary changes in VVL.
This code was being generated in both Vulkan-ValidationLayers
and Vulkan-ExtensionLayer. Further uses are on the horizon so
lets stop the copypasta.
Also, add functions to manipulate extension lists and pNext chains,
since many client layers have been doing that themselves.
