GitHub App event type security_advisory missing

[lukehinds]

The GitHub App can transmit dependabot events for security_advisory

Payload snippet:

{
  "action": "published",
  "security_advisory": {
    "ghsa_id": "GHSA-gggw-h5m3-x938",
    "cve_id": null,
    "summary": "Malware in eslint-plugin-artsy",
    "description": "Any computer that has this package installed or running should be considered fully compromised. All secrets and keys stored on that computer should be rotated immediately from a different computer. The package should be removed, but as full control of the computer may have been given to an outside entity, there is no guarantee that removing the package will remove all malicious software resulting from installing it.",
    "severity": "critical",
    "identifiers": [
      {
        "value": "GHSA-gggw-h5m3-x938",
        "type": "GHSA"
      }
    ],

Receiving this event results in unknown X-Github-Event in message: security_advisory

This needs to be added as an event type, with message mapping

Have a PR ready to add this.