Allow importing externally allocated memory into the allocator

[i509VCB]

It may be desired to import memory from an os object, such as a Win32 HANDLE, Linux Dmabuf, Fuchsia VMO, etc.

Ideally the Vulkan allocator could provide a way to unsafely create a memory block from some VkDeviceMemory and some auxiliary information (such as the size of the block, memory type, etc).