feat: add missing OAuth2Credentials class

lib/definitions.dart

@@ -14,6 +14,7 @@ export 'src/definitions/credentials/basic_credentials.dart';
 export 'src/definitions/credentials/bearer_credentials.dart';
 export 'src/definitions/credentials/credentials.dart';
 export 'src/definitions/credentials/digest_credentials.dart';
+export 'src/definitions/credentials/oauth2_credentials.dart';
 export 'src/definitions/credentials/psk_credentials.dart';
 export 'src/definitions/thing_description.dart';
 export 'src/definitions/thing_model.dart';

lib/src/core/consumed_thing.dart

@@ -11,6 +11,7 @@ import '../definitions/credentials/basic_credentials.dart';
 import '../definitions/credentials/bearer_credentials.dart';
 import '../definitions/credentials/credentials.dart';
 import '../definitions/credentials/digest_credentials.dart';
+import '../definitions/credentials/oauth2_credentials.dart';
 import '../definitions/credentials/psk_credentials.dart';
 import '../definitions/data_schema.dart';
 import '../definitions/form.dart';
@@ -19,6 +20,7 @@ import '../definitions/security/apikey_security_scheme.dart';
 import '../definitions/security/basic_security_scheme.dart';
 import '../definitions/security/bearer_security_scheme.dart';
 import '../definitions/security/digest_security_scheme.dart';
+import '../definitions/security/oauth2_security_scheme.dart';
 import '../definitions/security/psk_security_scheme.dart';
 import '../definitions/security/security_scheme.dart';
 import '../definitions/thing_description.dart';
@@ -89,6 +91,9 @@ class ConsumedThing implements scripting_api.ConsumedThing {
       } else if (securityDefinition is BearerSecurityScheme &&
           credentials is BearerCredentials) {
         securityDefinition.credentials = credentials;
+      } else if (securityDefinition is OAuth2SecurityScheme &&
+          credentials is OAuth2Credentials) {
+        securityDefinition.credentials = credentials;
       }
     }
   }

lib/src/definitions/credentials/oauth2_credentials.dart

@@ -0,0 +1,24 @@
+// Copyright 2022 The NAMIB Project Developers. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+//
+// SPDX-License-Identifier: BSD-3-Clause
+
+import 'credentials.dart';
+
+/// [Credentials] used for the `OAuth2SecurityScheme`.
+class OAuth2Credentials extends Credentials {
+  /// The [identifier] associated with these [OAuth2Credentials].
+  String identifier;
+
+  /// The optional secret for these [OAuth2Credentials].
+  String? secret;
+
+  /// A JSON string representation of OAuth2 credentials.
+  ///
+  /// Used to store obtained credentials from an authorization server.
+  String? credentialsJson;
+
+  /// Constructor.
+  OAuth2Credentials(this.identifier, [this.secret]) : super("oauth2");
+}

lib/src/definitions/security/oauth2_security_scheme.dart

@@ -4,7 +4,7 @@
 //
 // SPDX-License-Identifier: BSD-3-Clause
 
-import '../credentials/credentials.dart';
+import '../credentials/oauth2_credentials.dart';
 import 'helper_functions.dart';
 import 'security_scheme.dart';
 
@@ -41,10 +41,7 @@ class OAuth2SecurityScheme extends SecurityScheme {
   final List<String> _parsedJsonFields = [];
 
   @override
-  final Credentials? credentials = null;
-
-  // TODO(JKRhb): Check whether credentials should be used with this
-  //              Security Scheme.
+  OAuth2Credentials? credentials;
 
   /// Constructor.
   OAuth2SecurityScheme(this.flow,