Adds extensive integration tests that exercise the various ways of finding
the Vulkan-Headers.
vk-bootstrap should now use the Vulkan-Headers or Vulkan::Headers targets if
they were already defined (such as is the case of FetchContent), and will
look for the VulkanHeaders package and Vulkan package as a fallback, and will
FetchContent Vulkan-Headers directly if that fails.
This should make integration seamless with the various ways vulkan-headers
is acquired.
The vk-bootstrap-vulkan-headers target was dropped in favor of directly
linking to Vulkan-Headers (creating a target by that name if none exists).
