FR: Expose git-mirrors code as a sub-command?

I sometimes have builds that need to clone other git repos. I want those to be as fast as the git-mirrors feature that buildkite thoughtfully provide because waiting for things costs compute and engineering-time.

I would find it exceedingly useful if there were a buildkite-agent checkout-with-mirror sub-command which would allow me to do that clone via the git-mirrors experiment that the BK agent has, since building that inside bash is a pain especially since I already know it exists within BK.

That might look like

buildkite-agent checkout <git URL>
  --git-mirrors-path (defaults to same environment variable as in `--start`/config)
  --git-clone-flags (defaults to same environment variable as in `--start`/config)

Alternatively, allow a step to clone multiple repos…?

...
command: foo/build.sh
additional_repos:
  git:
    git@github.com:my-org/foo.git:
      into: foo
      depth: 1
...