Add `ci-publish` recipe to allow publishing a workspace member from
GitLab CI to crates.io.
Ensures that `CARGO_REGISTRY_TOKEN` is set to provide an auth token for
crates.io, that the tag provided by `CI_COMMIT_TAG` contains a valid
workspace member and that the version matches the workspace member's
metadata.

Signed-off-by: David Runge <dvzrv@archlinux.org>