Enable VK_EXT_index_type_uint8 Vulkan extension if supported by
VkDevice.

Bug: angleproject:4405
Change-Id: I84d030497898c5944a36d9a88a31e7377ccd5e9e
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2082391
Commit-Queue: Xiaoxuan Liu <xiaoxuan.liu@arm.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>