discoveryapis_generator: use a common userAgent across all libraries

Also fix the directives_ordering lint
diff --git a/generated/googleapis/lib/gamesconfiguration/v1configuration.dart b/generated/googleapis/lib/gamesconfiguration/v1configuration.dart
index 5cc47fd..58295bd 100644
--- a/generated/googleapis/lib/gamesconfiguration/v1configuration.dart
+++ b/generated/googleapis/lib/gamesconfiguration/v1configuration.dart
@@ -2,7 +2,6 @@
 
 // ignore_for_file: camel_case_types
 // ignore_for_file: comment_references
-// ignore_for_file: directives_ordering
 // ignore_for_file: file_names
 // ignore_for_file: library_names
 // ignore_for_file: lines_longer_than_80_chars
@@ -18,13 +17,15 @@
 
 library googleapis.gamesConfiguration.v1configuration;
 
-import 'dart:core' as core;
 import 'dart:async' as async;
 import 'dart:convert' as convert;
+import 'dart:core' as core;
 
 import 'package:_discoveryapis_commons/_discoveryapis_commons.dart' as commons;
 import 'package:http/http.dart' as http;
 
+import '../src/user_agent.dart';
+
 export 'package:_discoveryapis_commons/_discoveryapis_commons.dart'
     show
         ApiRequestError,
@@ -36,8 +37,6 @@
         PartialDownloadOptions,
         ByteRange;
 
-const userAgent = 'dart-api-client gamesConfiguration/v1configuration';
-
 /// The Google Play Game Services Publishing API allows developers to configure
 /// their games in Game Services.
 class GamesConfigurationApi {