Add authentication type to AuthenticateResponse (#5116)

elastic/elasticsearch#61247 added a new
"authentication_type" field to the response of
"GET _security/_authenticate". This commit adds this as a property of
the `AuthenticateResponse` type.

Contributes to meta issue #5096

Author: stevejgordon

src/Nest/XPack/Security/Authenticate/AuthenticateResponse.cs

@@ -40,5 +40,8 @@ public class AuthenticateResponse : ResponseBase
 
 		[DataMember(Name = "lookup_realm")]
 		public RealmInfo LookupRealm { get; internal set; }
+
+		[DataMember(Name = "authentication_type")]
+		public string AuthenticationType { get; internal set; }
 	}
 }