Provide couper via devcontainer feature

[malud]

See https://code.visualstudio.com/blogs/2022/09/15/dev-container-features

So we don't have to arch pick the correct download link.

[malud]

done.

"features": {
  "ghcr.io/coupergateway/features/couper": {}
}

[malud]

also via public index: devcontainers/devcontainers.github.io#614