[Feature Request] Allow using k8s secret for `erlangCookie`

The erlang cookie for interacting with rabbitmq is currently provided in plaintext. The upstream rabbitmq chart allows this to be specified by an `existingErlangSecret` and `existingErlangSecretKey` field. This allows users to manage helm charts with tools like gitops where helm values are checked into repositories. Without this, some secrets are exposed in the clear.