Introduce camera parameters for ui.scene

[falkoschindler]

As an extension to PR #2890, this PR makes perspective and orthographic cameras configurable, such that field-of-view (for perspective cameras) as well as the volume size (for orthographic cameras) can be configured:

with ui.scene(camera=ui.scene.perspective_camera(fov=50)) as scene:
    scene.box()
with ui.scene(camera=ui.scene.orthographic_camera(size=5)) as scene:
    scene.box()