Ability to select passthrough video configuration

[rockwalrus]

Many devices with passthrough video provide multiple device configurations that the developer can select between, providing different frame rates, resolutions, and even the direction the camera is facing. (See, for example, ArCamerConfig in ARCore.) It would be useful to be able to select which configuration is used by a session. The API could closely resemble the device capability and constraints system of the Media Streams API.