Allow `certSecretRef` to be provided as a `Secret` in a more standard way

[zzvara]

cert-manager, traefik and other tools, as I observed create certificates in secrets with the following keys tls.key, tls.crt. However, the image-reflector-controller assumes certFile and keyFile. We then have to copy-paste certificate secrets and reformat them to the certFile and keyFile key mapping in order to obey the image-reflector-controller.