Add dangerbot job

What does this MR do?

Adds danger-bot for this repo. Copied from ci-templates. The danger api token should be inherited from the group.

danger-review job status:

• initial pipeline run without labels: https://gitlab.com/gitlab-org/security-products/security-report-schemas/-/pipelines/671505555
• pipeline run after labels added: https://gitlab.com/gitlab-org/security-products/security-report-schemas/-/pipelines/671510325

What are the relevant issue numbers?

n/a

Checklist

• Ensure changes can be built upon without requiring a breaking change, see Building for Extensibility.
• Review and add/update tests for this feature/bug.
• Add an entry to the CHANGELOG if required, with the appropriate version. See Classifying Changes.
• Assign the MR to the appropriate person/people for review.