Add a ray-tracing backend

Ray-tracing backends implement the RayTracingBackend contract in simtools.ray_tracing.backends.interface. A backend traces the requests made by ray-tracing applications and returns image and incidence-angle results. A backend that traces calibration light sources also implements LightSourceRayTracingBackend and provides camera_photons.

Implementation checklist

1. Add a backend module under simtools.ray_tracing.backends.

2. Implement simulate, psf_image, and incidence_angles with the parameter signatures in RayTracingBackend.

3. For calibration light tracing, implement camera_photons with the signature in LightSourceRayTracingBackend and set traces_light_sources to True.

4. Add the backend name and its module:class path to RAY_TRACING_BACKEND_CLASSES in simtools.constants. Imports are lazy, so optional dependencies are needed only when the selected backend is loaded.

5. Add a contract test that checks the backend methods and their signatures.