Add cluster_uuid to root of the cluster_state response (#3443)

(cherry picked from commit 2130ddf)

Author: Mpdreamz

src/Nest/Cluster/ClusterState/ClusterStateResponse.cs

@@ -14,6 +14,11 @@ public interface IClusterStateResponse : IResponse
 		[JsonProperty("state_uuid")]
 		string StateUUID { get; }
 
+		/// <summary>The Universally Unique Identifier for the cluster.</summary>
+		/// <remarks>While the cluster is still forming, it is possible for the `cluster_uuid` to be `_na_`.</remarks>
+		[JsonProperty("cluster_uuid")]
+		string ClusterUUID { get; }
+
 		[JsonProperty("version")]
 		long Version { get; }
 
@@ -42,6 +47,9 @@ public class ClusterStateResponse : ResponseBase, IClusterStateResponse
 
 		public string StateUUID { get; internal set; }
 
+		/// <inheritdoc cref="IClusterStateResponse.ClusterUUID"/>
+		public string ClusterUUID { get; internal set; }
+
 		public long Version { get; internal set; }
 
 		public IReadOnlyDictionary<string, NodeState> Nodes { get; internal set; } = EmptyReadOnly<string, NodeState>.Dictionary;