Open issue based on Pagerduty notification

We should consider enabling Pagerduty incidents to trigger the creation of GitLab issues. This would enable a corresponding issue/incident to be opened within GitLab, which can be used to track relevant MR's, Issues, and other content.

Pagerduty has support for webhook notifications, with a rich set of content: https://v2.developer.pagerduty.com/docs/webhooks-v2-overview#incident-details

We can use this as an initial integration point, or we could instead build a dedicated integration and contribute it to Pagerduty.