multiple trivial changes for "extras" models

[victorca25]

This is a small PR is to add a few requests regarding the "extra" models available. Most of the changes are relatively trivial and don't impact the application logic. These are the changes:

• added support for realESRGAN architecture with additional up_conv block ("8x") (https://github.com/AUTOMATIC1111/stable-diffusion-webui/issues/3536).
• automatically move BSRGAN models from the previous BSRGAN directory to the ESRGAN directory, to avoid BSRGAN model from "disappearing" when updating the code. (https://github.com/AUTOMATIC1111/stable-diffusion-webui/issues/3551, https://github.com/AUTOMATIC1111/stable-diffusion-webui/discussions/3596).
• added basic Nearest Neighbor upscaling option when working with pixel-art and avoid blurry scaling (Discord request).
• change maximum upscaling size to 8, to accomodate for larger upscale factor models.
• set the default upscaling value to 4, the "native" upscaling factor of most models.
• allow use of 1x scale models like ESRGAN, ScuNET, and others that are mainly used for denoising and deblurring without changing images scales. (https://github.com/AUTOMATIC1111/stable-diffusion-webui/issues/3526).