vulkan: Add a base struct for descriptor set layouts

vulkan: Add a base struct for descriptor set layouts

There's some tricky stuff in here with properly handling Vulkan
allocation scopes and reference counting.  Probably best to do it once.
Also, this means that common code can now take references to descriptor
set layouts which seems useful.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17286>