feat: add command-line tool admiralctl join&unjoin

Summary

Currently, users need to manually create Secret and FederatedCluster resources through YAML files to add member clusters, and this file-based approach is not very convenient. Thanks to the work in #202, the command-line tool admiralctl has taken shape. In this proposal, I am building upon this work by introducing a new subcommand, admiralctl join, corresponding to #260. With this command, users can add member clusters via the command line.

Motivation

With the command-line tool, users can conveniently add member clusters without the need to manually create Secret and FederatedCluster resources.

Proposal

Command line tool framework selection

Similar to #202, utilizing cobra as the framework for the command-line tool.

`admiralctl join` subcommand

Retrieve the kubeconfig file of the member cluster and extract information such as server endpoint, CA, cert, key, and bearer token.
Validate the legitimacy of the member cluster:
- Check if the control-plane nodes of the member cluster are Ready.
- Verify if the member cluster has already joined Kubeadmiral (by checking the existence of the kube-admiral-system namespace and the annotation of it).
Create Secret and FederatedCluster resources through the Clientset.

Command design

./admiralctl join --help
Join a member cluster to Kubeadmiral control plane.

 If the control plane of the member cluster is not ready or has already joined, this command will do nothing.

Examples:
  # Join cluster1 to Kubeadmiral by kubeconfig
  admiralctl join cluster1 --cluster-kubeconfig=<CLUSTER_KUBECONFIG_PATH>

  # Support to use '--api-endpoint' to overwrite the member cluster's api-endpoint
  admiralctl join cluster1 --cluster-kubeconfig=<CLUSTER_KUBECONFIG_PATH> --api-endpoint=<CLUSTER_API_ENDPOINT>

  # Support to use '--use-service-account' to determine whether create a new ServiceAccount when join
  admiralctl join cluster1 --cluster-kubeconfig=<CLUSTER_KUBECONFIG_PATH> --use-service-account=false

  # Support to use '--cluster-context' to specify the context of member cluster
  admiralctl join cluster1 --cluster-kubeconfig=<CLUSTER_KUBECONFIG_PATH> --cluster-context=<CLUSTER_CONTEXT>

Options:
    --api-endpoint='':
        api-endpoint of the member cluster.

    --cluster-context='':
        Context name of member cluster in kubeconfig.

    --cluster-kubeconfig='':
        Path of the member cluster kubeconfig.

    --use-service-account=true:
        Whether creates a new ServiceAccount when join, corresponding to FederatedCluster.spec.useServiceAccount.
        If you set 'false', BearerToken should be in the kubeconfig.

Usage:
  admiralctl join <FCLUSTER_NAME> --cluster-kubeconfig <CLUSTER_KUBECONFIG_PATH> [options]

`admiralctl unjoin` subcommand

Validate the legitimacy of the member cluster:
- Verify if the cluster has already joined Kubeadmiral
- Check if the secret of the FederatedCluster is existed.
Delete the FederatedCluster resource from Kubeadmiral.

Command design

./admiralctl unjoin --help
Unjoin a FederatedCluster from Kubeadmiral federation.

 If the the federated cluster has not joined the federation, this command will do nothing.

Examples:
  # Unjoin cluster1 from Kubeadmiral federation
  admiralctl unjoin cluster1

Usage:
  admiralctl unjoin <FCLUSTER_NAME> [options]

kubewharf / kubeadmiral