Configure monitoring authentication proxy for instances which require authentication

Configure an authentication proxy within the monitoring module's variables if the cloud run instance does not allow unauthenticated access.

Part of https://gitlab.developers.cam.ac.uk/uis/devops/iam/deploy-identity/-/issues/23.

Tested within https://gitlab.developers.cam.ac.uk/uis/devops/iam/deploy-identity/-/merge_requests/30