Allow Rancher to provisioning registries.yaml with auth key values instead of username and password

[btat]

SURE-6308

There's currently an undocumented K8s secret type rke.cattle.io/auth-config that would provisioning as described in the title. We should document how to use it, so people could utilize this secret before the UI feature is ready. This way we could pre-create it, and just supply the secret via the yaml.