Support joining/unjoining member cluster via admiralctl command-line tool

[mrlihanbo]

What would you like to be added?

Enhance the admiralctl command-line tool to support joining or unjoining member cluster.

Why is this needed?

Currently, users need to create auth secret and FederatedCluster resource manually. when joining a member cluster. It will greatly improve the convenience for users to use kubeAdmiral if we provide the joining/unjoining command-line tool.