Support for VK_KHR_portability_subset Devices

[AustinJ235]

Adds support for VK_KHR_portability_subset devices.

Changelog:

- **Breaking** Removed `DeviceExtensions::required_extensions()`
- **Breaking** Removed `PhysicalDevice::required_extensions()`
- `InstanceCreateInfo` now has `enumerate_portability` field.

[Rua]

What should Vulkano do with the enumerate_portability field if the khr_portability_enumeration extension isn't available? The previous default behaviour, before the extension was introduced, was equivalent to setting it to true. Should Vulkano enforce that the flag is always true for such cases, or should we explicitly document that Vulkan(o) behaves as if it's set to true and the user-set value is ignored?

[AustinJ235]

What should Vulkano do with the enumerate_portability field if the khr_portability_enumeration extension isn't available? The previous default behaviour, before the extension was introduced, was equivalent to setting it to true. Should Vulkano enforce that the flag is always true for such cases, or should we explicitly document that Vulkan(o) behaves as if it's set to true and the user-set value is ignored?

Documentation add with a6843ef