Add a flag to enable the VK_KHR_timeline_semaphore extension.
Add a flag to enable the VK_KHR_timeline_semaphore extension.

Also attempts to load the `VK_LAYER_KHRONOS_timeline_semaphore` layer.
Future changes will build that from `third_party/vulkan_extensionlayer`.
Unlike with SwiftShader, we want to distribute the extension layer along
with IREE in applications, so that'll be an in-tree integration and not a
self-service bash script / list of instructions.

Closes https://github.com/google/iree/pull/966

COPYBARA_INTEGRATE_REVIEW=https://github.com/google/iree/pull/966 from ScottTodd:vk-timeline-semaphore 0403d1a24e06490e86cb509790780b56c6c5ce46
PiperOrigin-RevId: 298994135

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