[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.
I am conflicted on the category we should use here, ping @uboness .. `cluster:monitor` ?  or `internal:`? 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. Thoughts?