feat(safe-settings): initial commit of GitHub's safe-settings utility

[dafyddj]

• The workflow expects the following repository variable:
  SAFE_SETTINGS_APP_ID: Your GitHub App ID

  and the repository secret:
  SAFE_SETTINGS_PRIVATE_KEY: The Base64 encoded private key generated
  for the above GitHub App

  By default, runtime configuration and desired GitHub settings to be
  applied should be placed in the directory safe-settings

[dafyddj]

The settings.yml file applies to all in-scope repositories. (Currently template-formula)
This will eventually need to be differentiated and expanded upon for sub-orgs and individual repos as we proceed with the safe-settings rollout.