panvk: implement more extensions

This implements VK_IMAGE_CREATE_EXTENDED_USAGE_BIT for KHR_maintenance2, as well as exposes KHR_relaxed_block_layout, which were already KHR_bind_memory2 implemented.

Edit: went ahead and added KHR_external_semaphore, KHR_external_semaphore_capabilities, KHR_external_memory, KHR_external_memory_capabilities, KHR_external_fence and KHR_external_fence_capabilities as well. I think some of these are implemented by the runtime, but the details are kinda hazy to me ;)