feat: customize target Device Pixel Ratios

[sherwinski]

Description

Related to https://github.com/imgix/js-core#fixed-image-rendering

Add a device_pixel_ratios option to the https://github.com/imgix/imgix-rb#fixed-image-rendering 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.