aks connectedk8s connect should allow connecting to existing arc kubernetes resources and use own key-pair

### Extension name (the extension in question)
`connectedk8s`

### Description of issue (in as much detail as possible)

Hello 👋🏼
We're trying to add support for Arc Kubernetes in the AzureRM Provider for Terraform, unfortunately there is no means of connecting an Arc Kubernetes resource that was created by Terraform to a Kubernetes cluster, it will remain with status `Connecting`.

I attempted the following without success:
1. Command `az connectedk8s connect --name arc-kubernetes-resource-name --resource-group resource-group-name`

This doesn't work because the command creates the Arc Kubernetes resource for you, so it will fail if a resource already exists with the same name.

2. Pull and deploy the helm chart that `az connectedk8s connect ...` runs

I used the same values that are passed from the `az connectedk8s` CLI to the Helm chart, but substituted my own RSA key pair generated via `openssl`, the deployment fails with pods in `kube-aad-proxy`and `config-agent` ending in a `CrashLookBackOff`.

Ideally the connect command should allow the user to connect a cluster to an existing Arc Kubernetes resource and to supply our own private key for the connection. Would this be possible or is there an alternative way of connecting an existing Arc Kubernetes resource to a cluster?