Support for DRM `syncobj` FDs in `vkGetSemaphoreKHR`/`vkImportSemaphoreFdKHR`

[DRM timeline sync objects](https://www.kernel.org/doc/html/v5.4/gpu/drm-mm.html?highlight=syncobj#drm-sync-objects) seem tailor-made for interop with Vulkan timeline semaphores. However, while it's possible to import these using [`vkImportSemaphoreFdKHR`](https://registry.khronos.org/VulkanSC/specs/1.0-extensions/man/html/vkImportSemaphoreFdKHR.html) and `VK_EXTERNAL_SEMAPHORE_HANDLE_TYPE_OPAQUE_FD_BIT` on some drivers, there are a few issues that make importing/exporting unergonomic:

- When importing, the `SYNC_FD` type seems more correct, and it's not clear in the spec which should be used in this case (unless I missed it). However, `SYNC_FD` is restricted to temporary binary semaphore imports, and does not allow permanent timeline semaphore imports.
- When exporting a semaphore as a FD using [`vkGetSemaphoreFdKHR`](https://registry.khronos.org/VulkanSC/specs/1.0-extensions/man/html/vkGetSemaphoreFdKHR.html), there's no way to distinguish what type of FD you're getting.

Should syncobjs be covered by an additional enum value in [`HandleTypeFlagBits`](https://registry.khronos.org/VulkanSC/specs/1.0-extensions/man/html/VkExternalSemaphoreHandleTypeFlagBits.html)? Or should the the spec/validation be relaxed to allow timeline semaphores to be imported/exported using `SYNC_FD`?