Add basic authentication

The config server is running with the same permissions as `dls-dasc`, so we shouldn't allow it to be used by anyone in DLS.

For now, the easiest solution is to add an ingress rule, using something like https://kubernetes.github.io/ingress-nginx/examples/auth/basic/. Each beamline cluster will need need valid credentials, using secrets.

## Acceptance Criteria
- Ingress rule added to helm chart
- Document how to give a new beamline cluster valid credentials