gatus: allow using an existing kubernetes secret for the config.yaml

[jessebot]

Summary

Adds a new externalConfigSecret to the gatus helm chart, so users can use an existing Kubernetes Secret. This is useful for if the config.yaml would contain a sensitive api key or URL, for instance for the alerts.

Checklist

• Tested and/or added tests to validate that the changes work as intended, if applicable.
• Updated documentation in README.md, if applicable.

[klaernie]

Just wondering:

the helm chart is already including a secret called gatus (name determined from names.fullname helper) into the environment, if secrets: true is set. Together with the environment interpolation in config.yaml this means there is already a way to securely store the sensitive data in a secret and reference it from the config in a configmap.

Why would there be the need to move the entire config into a secret?