Add restatectl command to add metadata store peer members

[tillrohrmann]

After #2412, we can expose a restatectl command to add/remove metadata store peers. This will allow operators to start by provisioning a multi node cluster and then increase the resilience of the metadata store by adding more peers to it. To simplify things, it is the responsibility of the user to retry if this operation fails. Hence, the operation should be idempotent.