Add VK_EXT_external_memory_metal #999

Job	Run time
Build the core-only spec, to try and catch ifdef errors in extension markup	52s
Verify repository license compliance	40s
Run consistency checks on the spec and XML sources	53s
Run Vulkan CTS framework tests to validate against XML changes	1m 49s
Build the vulkan specification and generate any associated files (such as vulkan.h)	7m 8s
Build a test spec and check against expectation, to make sure the spec toolchain works as expected	1m 16s
Generate the vulkan C++ header (vulkan.hpp)	1m 5s
Compile a simple test program that uses vulkan.h	10s
Generate Rust bindings (Ash crate)	40s
Build-test Rust bindings (Ash crate)	37s
Compile a simple test program that uses vulkan.hpp	28s
	15m 38s

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