FB foveation extensions are missing some useful information

XR_FB_foveation_vulkan is not clear on what format the image in XrSwapchainImageFoveationVulkanFB has nor provides a way to set or query it.
The vulkan specification only says:
`VK_FORMAT_FEATURE_FRAGMENT_DENSITY_MAP_BIT_EXT must be supported for the following formats if the fragment density map feature is enabled:
VK_FORMAT_R8G8_UNORM`
Which I don't think excludes other formats?

Also there is no way to specify the desired foveation profile on swapchain creation, but at the same time I have not found any information on what the default is (I am assuming it's just no foveation at all). When implementing it, it seemed a bit strange that there is no way to unset it, but at the same time it can only be set after the swap-chain creation.

Review XR_FB_foveation_vulkan and XrSwapchainImageFoveationVulkanFB alongside the cited Vulkan fragment density map requirement. Determine and document the supported image format, default foveation profile, and how a profile can be unset or selected at swapchain creation; done when the specification answers these questions.

Written by the indexing model from the issue text.