Support multiple secrets for auth-tls-secret annotation

[hoeg]

Currently the nginx.ingress.kubernetes.io/auth-tls-secret annotation takes a single secret with a key named ca.crt (https://kubernetes.github.io/ingress-nginx/user-guide/nginx-configuration/annotations/#client-certificate-authentication). Kubernetes secrets can at most be 1MB (https://github.com/kubernetes/kubernetes/issues/19781) so this leaves us with an upper limit on the number of certificates we can use for a single ingress resource. We would like to have the possibility to define a list of secrets instead of a single secret such that we are not bound by the upper limit size of a kubernetes secret.

We will be happy to submit a pull request with this feature 🚀