Add NVidia AI denoiser support

As a user, I would like to have the option to use the [NVidia AI denoiser](https://github.com/DeclanRussell/NvidiaAIDenoiser) instead of the Intel Open Image denoiser. The way the denoiser operates seems functionally similar except for the program command names (e.g. using -n for normal instead of -nrm) and taking .png images instead of .pfm images.

Start by tracing the existing Intel Open Image denoiser integration and how it builds the external command and handles .pfm images. Add a selectable NVIDIA AI denoiser path using the command differences described in the issue and .png inputs, then verify both denoiser options still operate correctly.

Written by the indexing model from the issue text.