[Feature] Add support for automatic image scaling with client hints

# **Feature Request**

## **Is your feature request related to a problem? Please describe.**

I would like to specify a [Cloudinary crop mode](https://cloudinary.com/documentation/resizing_and_cropping#examples_of_applying_different_crop_modes) from my netlify.toml file; the current default value is `limit` and cannot be changed via netlify.toml.

I would also like to be able to specify `"auto"` as the value for maxSize.height and/or maxSize.width so that I can [scale images automatically using Client Hints](https://cloudinary.com/blog/automatic_responsive_images_with_client_hints).

## **Describe the solution you'd like**

Allowing a string to be used as the value of maxSize.height and maxSize.width would allow users to:

1. [scale image widths automagically](https://cloudinary.com/blog/automatic_responsive_images_with_client_hints#how_to_scale_an_image_with_automatic_width)
2. [dictate how big the "jumps" between automagic image widths should be](https://cloudinary.com/blog/automatic_responsive_images_with_client_hints#advanced_w_auto_usage), reducing cache misses

Users should be able to specify maxSize.crop to help with optimizing the final aesthetic result of an automatically scaled image on a site.

## **Describe alternatives you've considered**

I haven't considered alternatives, but I'm open to suggestions~