feat(definitions): add CredentialsScheme interface

lib/definitions.dart

@@ -16,5 +16,6 @@ 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/security/credentials_scheme.dart';
 export 'src/definitions/thing_description.dart';
 export 'src/definitions/thing_model.dart';

lib/src/definitions/security/apikey_security_scheme.dart

@@ -5,12 +5,13 @@
 // SPDX-License-Identifier: BSD-3-Clause
 
 import '../credentials/apikey_credentials.dart';
+import 'credentials_scheme.dart';
 import 'helper_functions.dart';
 import 'security_scheme.dart';
 
 /// API key authentication security configuration identified by the Vocabulary
 /// Term `apikey`.
-class ApiKeySecurityScheme extends SecurityScheme {
+class ApiKeySecurityScheme extends SecurityScheme implements CredentialsScheme {
   @override
   String get scheme => "apikey";
 

lib/src/definitions/security/basic_security_scheme.dart

@@ -5,12 +5,13 @@
 // SPDX-License-Identifier: BSD-3-Clause
 
 import '../credentials/basic_credentials.dart';
+import 'credentials_scheme.dart';
 import 'helper_functions.dart';
 import 'security_scheme.dart';
 
 /// Basic Authentication security configuration identified by the Vocabulary
 /// Term `basic`.
-class BasicSecurityScheme extends SecurityScheme {
+class BasicSecurityScheme extends SecurityScheme implements CredentialsScheme {
   @override
   String get scheme => "basic";
 

lib/src/definitions/security/bearer_security_scheme.dart

@@ -5,12 +5,13 @@
 // SPDX-License-Identifier: BSD-3-Clause
 
 import '../credentials/bearer_credentials.dart';
+import 'credentials_scheme.dart';
 import 'helper_functions.dart';
 import 'security_scheme.dart';
 
 /// Bearer Token security configuration identified by the Vocabulary Term
 /// `bearer`.
-class BearerSecurityScheme extends SecurityScheme {
+class BearerSecurityScheme extends SecurityScheme implements CredentialsScheme {
   @override
   String get scheme => "bearer";
 

lib/src/definitions/security/credentials_scheme.dart

@@ -0,0 +1,13 @@
+// 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/credentials.dart';
+
+/// Interface to ensure that a given `SecurityScheme` has a [credentials] field.
+abstract class CredentialsScheme {
+  /// The credentials assigned to this [CredentialsScheme].
+  Credentials? get credentials;
+}

lib/src/definitions/security/digest_security_scheme.dart

@@ -5,12 +5,13 @@
 // SPDX-License-Identifier: BSD-3-Clause
 
 import '../credentials/digest_credentials.dart';
+import 'credentials_scheme.dart';
 import 'helper_functions.dart';
 import 'security_scheme.dart';
 
 /// Digest Access Authentication security configuration identified by the
 /// Vocabulary Term `digest`.
-class DigestSecurityScheme extends SecurityScheme {
+class DigestSecurityScheme extends SecurityScheme implements CredentialsScheme {
   @override
   String get scheme => "digest";
 

lib/src/definitions/security/oauth2_security_scheme.dart

@@ -5,13 +5,14 @@
 // SPDX-License-Identifier: BSD-3-Clause
 
 import '../credentials/oauth2_credentials.dart';
+import 'credentials_scheme.dart';
 import 'helper_functions.dart';
 import 'security_scheme.dart';
 
 /// OAuth 2.0 authentication security configuration for systems conformant with
 /// RFC 6749, RFC 8252 and (for the device flow) RFC 8628, identified by the
 /// Vocabulary Term `oauth2`.
-class OAuth2SecurityScheme extends SecurityScheme {
+class OAuth2SecurityScheme extends SecurityScheme implements CredentialsScheme {
   @override
   String get scheme => "oauth2";
 

lib/src/definitions/security/psk_security_scheme.dart

@@ -5,12 +5,13 @@
 // SPDX-License-Identifier: BSD-3-Clause
 
 import '../credentials/psk_credentials.dart';
+import 'credentials_scheme.dart';
 import 'helper_functions.dart';
 import 'security_scheme.dart';
 
 /// Pre-shared key authentication security configuration identified by the
 /// Vocabulary Term `psk`.
-class PskSecurityScheme extends SecurityScheme {
+class PskSecurityScheme extends SecurityScheme implements CredentialsScheme {
   @override
   String get scheme => "psk";