Add Support for GLTF Cameras on import

[robertlong]

Cameras are currently deserialized into the GLTFRoot object, but not translated into a Unity Camera. There should likely be settings to turn off creating Unity Camera objects, set a camera as the main camera, etc.

GLTF 2.0 Animation Spec https://github.com/KhronosGroup/glTF/tree/2.0/specification/2.0#cameras

Unity Camera Component https://docs.unity3d.com/ScriptReference/Camera.html