+ use per-client DNS servers

AGHTechDoc.md

@@ -669,6 +669,7 @@ Response:
 				key: "value"
 				...
 			}
+			upstreams: ["upstream1", ...]
 		}
 	]
 	auto_clients: [
@@ -703,6 +704,7 @@ Request:
 		safesearch_enabled: false
 		use_global_blocked_services: true
 		blocked_services: [ "name1", ... ]
+		upstreams: ["upstream1", ...]
 	}
 
 Response:
@@ -732,6 +734,7 @@ Request:
 			safesearch_enabled: false
 			use_global_blocked_services: true
 			blocked_services: [ "name1", ... ]
+			upstreams: ["upstream1", ...]
 		}
 	}