[CLIENT] Add internal liveness action

This commit adds a very lightweight action to the transport
serivce that allows to fetch clustername and the discovery node
from a node. This is used by transport clients to test liveness of
a node without using the nodesinfo API which can be blocking if management
threadpools are busy.
@javanna @s1monw it's certainly not internal... I'd name categorize it under `cluster:monitor`

> It doesn't seem internal to me since it's called by the transport client but it is not exposed to java api which makes it somehow special.

@javanna what do you mean by that? how can it not be exposed to the Java API?