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

[gaktive]

Internal reference: SURE-6308

Paraphrasing @nwmac,

When creating an RKE2 Cluster, in the 'Cluster Configuration' section at the bottom, we have the 'Registries' tab. Within this, you can enable registry configuration and as part of this, choose to create a username and password - a k8s secret (of type HTTP Basic Auth) will be created and the secret name used.

This request is about allowing the user to create a k8s secret of type rke.cattle.io/auth-config (this is not well documented) with the secret's auth key set to the base64 encoded concatenated username and password (see: https://github.com/rancher/rancher/blob/main/pkg/apis/rke.cattle.io/v1/registries.go#L32).

I'd suggest in the UI, we add a 2nd type of secret in the dropdown box 'Create an RKE Auth Config Secret (base64 encoded)' - this would take the username and password and on secret creation, create a different type of secret with the username and password concatenated by a colon and base64 encoded.

I would suggest that the base64 encoded secret option is the default.