Basic alerting

This adds optional basic uptime alerting to the cloud run app. Existing clients of the main module won't see any change, since the new resources depend on an optional variable being set.

See https://gitlab.developers.cam.ac.uk/uis/devops/raven/shibboleth-metadata-deployment/-/issues/8 and https://gitlab.developers.cam.ac.uk/uis/devops/tls-certificates/google-cloud-deploy/-/issues/8