[Tooling] Setup Dangermattic

[iangmaia]

This PR adds Danger / Dangermattic to the project: the Buildkite pipeline as well as the Github Action workflow to trigger it based on events such as on label / reviewers updates.

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• I have considered if this change warrants release notes and have added them to the appropriate section in the CHANGELOG.md if necessary.

[AliSoftware]

The rules applied here looks good to me and appropriate to how we use this repo 👍

[AliSoftware]

Such a change in steps labels will require updating the branch protection settings on GitHub with the new derived names for the required checks.

Given the long names Buildkite already ends up generating on the existing GitHub checks (e.g. buildkite/ci-toolkit-buildkite-plugin/swift-standalone-swift-package-autodetect) and how using :emoji: codes like here will also lead to long labels (like buildkite/ci-toolkit-buildkite-plugin/radioactive-sign-danger-pr-check)… I think this could be a good occasion to use this repo as guinea pig for paaHJt-78h-p2, as we could consider:

• Turning off the publish_commit_status setting on the pipeline (the one that makes Buildkite generate status based on label names)
• Instead declare notify attributes (with e.g. {github_commit_status: { context: "Danger - PR Check" } }) explicitly for every single step in the pipeline.yml

PS: Given all of this, you might want to revert this commit from the time being and instead apply those changes in a separate PR instead.

[mokagio]

Thanks for setting this up @iangmaia . Love having automation working to help me remember things.