Add support for GitLab Code Quality reports

This commit adds support for enabling [GitLab Code Quality](https://docs.gitlab.com/ee/ci/testing/code_quality.html)
reports. I've not enabled it by default (in `config_defaults.yml`), but
this is something that perhaps we could do at a later date following any
user feedback.

Meanwhile, the following module `.sync.yml` content can be used to turn
it on.

```yaml
.gitlab-ci.yml:
  custom:
    ruby_versions:
      '2.7.2':
        code_quality: true
```