[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.
the `execute` method requires an `Action` object, which this action doesn't have, as it is only a transport handler and that is way I'm saying it is not exposed through the `Client` object. That said, it is not an internal action anyway, I agree, `cluster:monitor/nodes/liveness` is fine with me.