[Features] support shell scripts during memberJoin

[loomts]

What is the user interaction of your feature developer can customize shell scripts when member join into the cluster

Is your feature request related to a problem? Please describe. etcd-addon need to handle member join scenario, such as

• attach one etcd pod or startup a job pod and exec etcdctl --endpoints="old-peer-fqnd:2379" member add --peer-urls="new-peer1:new-peer1-fqnd:2380,new-peer2:new-peer2-fqnd:2380..."
• change the config file of new etcd member, from initial-cluster-state: new to initial-cluster-state: existing

Describe the solution you'd like

1. select one role of cluster or startup a job pod and do some preparation
2. attach new member and grant them the authority to override the startup command