Subject-segmentation placeholder operation. Cloudflare's segment=foreground
isolates the subject and replaces the background with transparency.
The op is currently carried through the pipeline as a no-op so requests
using segment=foreground do not 400. A real implementation will land
alongside :image_vision background-removal wire-up (see TODO.md).
Summary
Types
@type kind() :: :foreground
@type t() :: %Image.Plug.Pipeline.Ops.Segment{kind: kind()}