group step: key attribute silently dropped during YAML serialisation

[JonCubed]

Problem

It is not possible to target a group step as a dependency target via depends_on. While both the Buildkite group step spec and the plugin README document key as a supported attribute, the key field is silently dropped from group steps during YAML serialisation.

Example config:

watch:
  - path: services/
    config:
      group: CI/CD Infrastructure
      key: group:cicd
      steps:
        - label: Deploy
          command: echo deploy

Generated output (incorrect):

steps:
  - group: CI/CD Infrastructure
    steps:
      - label: Deploy
        command: echo deploy

key: group:cicd is missing from the group.

Root cause

The Group struct used during YAML marshalling only carried Label and Steps. When MarshalYAML converted a Step with Group != "" into a Group, any Key on the step was discarded.