Add VK_EXT_external_memory_metal #887

Job	Run time
Build the core-only spec, to try and catch ifdef errors in extension markup	47s
Build the vulkan specification and generate any associated files (such as vulkan.h)	5m 37s
Verify repository license compliance	35s
Run Vulkan CTS framework tests to validate against XML changes	1m 32s
Build a test spec and check against expectation, to make sure the spec toolchain works as expected	1m 14s
Run various checker scripts on the spec and XML sources	48s
Generate the vulkan C++ header (vulkan.hpp)	0s
Generate Rust bindings (Ash crate)	0s
Compile a simple test program that uses vulkan.h	0s
Build-test Rust bindings (Ash crate)	0s
Compile a simple test program that uses vulkan.hpp	0s
	10m 33s

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