Add `tiup cluster pdctl` command

Feature Request

Is your feature request related to a problem? Please describe:

We use pd-ctl frequently. For example, to get a list of stores, we need to use:

tiup ctl:<cluster_version> pd -u http://<pd_address>:2379 store

In order to execute the command, I need to look up the cluster_version and the pd_address first, by something like:

tiup cluster display my_cluster

It is not very convenient.

Describe the feature you'd like:

Introduce a new command for tiup cluster, so that we can just:

tiup cluster pdctl <cluster_name> <commands>

like:

tiup cluster pdctl my_cluster store

Considering that only pd-ctl may omit the PD address, and other ctls (like tikv-ctl) still need to explicit assign an address, may be we can just introduce this shortcut for pd-ctl only.

In future we can also have a shortcut for etcdctl, if necessary:

tiup cluster etcdctl my_cluster get ...

Considering that etcd-ctl is rarely used, I think it is fine to just have pdctl.

Why the featue is needed:

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy:

Start by tracing the existing tiup cluster display command and the tiup ctl:<cluster_version> pd invocation described in the issue. Determine how the cluster name can supply the PD address and version, then add tiup cluster pdctl <cluster_name> <commands> with the example store operation working as the definition of done.