context: Initialize gulkan with Vk Ext strings
This removes gxr_context_new_from_gulkan to simplify initialization:
GulkanClient is to be initialized with Vulkan extensions
- decided by the application developer (e.g. xrdesktop)
- required by the XR runtime (e.g. OpenVR)
While instance extensions can be queried without prerequisites, querying the required Vk device extensions from OpenVR requires a Vk instance.
gxr_context_new_from_vulkan_extensions() now allows application developers to specify lists of Vk instance and Vk device extensions without having to worry about the XR runtimes' requirements.