feat: add oauth2 ui and authorzation code grant handling

packages/better_networking/lib/utils/auth/oauth2_utils.dart

@@ -0,0 +1,75 @@
+import 'dart:async';
+import 'dart:developer' show log;
+import 'dart:io';
+import 'package:flutter_web_auth_2/flutter_web_auth_2.dart';
+import 'package:oauth2/oauth2.dart' as oauth2;
+
+Future<oauth2.Client> oAuth2AuthorizationCodeGrantHandler({
+  required String identifier,
+  required String secret,
+  required Uri authorizationEndpoint,
+  required Uri tokenEndpoint,
+  required Uri redirectUrl,
+  required File credentialsFile,
+  String? state,
+  String? scope,
+}) async {
+  if (await credentialsFile.exists()) {
+    try {
+      final json = await credentialsFile.readAsString();
+
+      final credentials = oauth2.Credentials.fromJson(json);
+
+      if (credentials.accessToken.isNotEmpty && !credentials.isExpired) {
+        log('Using existing valid credentials');
+
+        return oauth2.Client(
+          credentials,
+          identifier: identifier,
+          secret: secret,
+        );
+      }
+    } catch (e) {
+      log('Error reading existing credentials: $e');
+    }
+  }
+
+  final grant = oauth2.AuthorizationCodeGrant(
+    identifier,
+    authorizationEndpoint,
+    tokenEndpoint,
+    secret: secret,
+  );
+
+  final authorizationUrl = grant.getAuthorizationUrl(
+    redirectUrl,
+    scopes: scope != null ? [scope] : null,
+    state: state,
+  );
+  log('Generated authorization URL: ${authorizationUrl.toString()}');
+  log('Expected redirect URL: ${redirectUrl.toString()}');
+
+  final uri = await FlutterWebAuth2.authenticate(
+    url: authorizationUrl.toString(),
+    callbackUrlScheme: redirectUrl.scheme,
+    options: const FlutterWebAuth2Options(useWebview: true),
+  );
+
+  try {
+    final client = await grant.handleAuthorizationResponse(
+      Uri.parse(uri).queryParameters,
+    );
+
+    log('OAuth2 authorization successful, saving credentials');
+
+    await credentialsFile.writeAsString(client.credentials.toJson());
+
+    return client;
+  } catch (e) {
+    log('Error handling authorization response: $e');
+
+    log('URI query parameters: ${Uri.parse(uri).queryParameters}');
+
+    rethrow;
+  }
+}