feat: customize target Device Pixel Ratios

# Description

> Related to https://github.com/imgix/js-core#fixed-image-rendering

Add a `devicePixelRatios` option to the https://github.com/imgix/imgix-php#fixed-width-images function to allow users to customize the target device pixel ratio(s) (`dpr`) when generating a fixed image. This will allow users to have more granular control over the fixed-size srcset values generated.