Add Support for GLTF Cameras on import

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

Start with the existing camera deserialization into GLTFRoot, then read the linked glTF 2.0 camera specification and Unity Camera documentation. Define how imported cameras become Unity Camera objects, including whether creation can be disabled and how the main camera is selected. Done means glTF cameras import into Unity with the agreed settings and behavior.

Written by the indexing model from the issue text.