Allow additional uptime checks

The Digital Admissions project has some additional uptime check requirements that would be good to implement using this shared module.

This issue is to refactor the code so that:

• Creation of a notification channel is opt-out and an existing one can be specified instead.
• A http_check can configure the content_matchers block.
• The auth_proxy module works when the target Cloud Run module uses IAM authentication and a custom healthcheck url is specified. Currently this fails as the Cloud Function code doesn't honour the fact that the audience parameter for the google.oauth2.id_token.fetch_id_token() function should be the root url for the Cloud Run, not the full url including the custom healthcheck endpoint.