Treat cluster state as key value response in 7.0

[Mpdreamz]

See: elastic/elasticsearch#38413

cluster state will be documented as key/value response

We should model this as such in 7.0 before we ship I reckon.

We can still ship objects to assist in the mapping but maybe not expose these directly on the response as properties that are part of the deserialization,

[codebrain]

Merged: #3824