Allow User to Select Image Type for Saved Images

### Feature Idea

Currently, ComfyUI saves all images as PNG. Other file types support metadata as well, including webp. Ideally there would be a setting in the settings menu to change the default image save type.

### Existing Solutions

I know custom nodes exist to tackle this but it would be nice to have a simple setting in the settings menu to change the default image save type.

### Other

_No response_

The issue names no files, tests, or entry points. Start by tracing the settings menu and the existing path that saves images as PNG, then determine how a selected default format should flow through saving. Done means the setting changes the default saved image type while preserving supported metadata formats.

Written by the indexing model from the issue text.