feat(protocol_interfaces): introduce ProtocolSubscription class

lib/src/core/protocol_interfaces.dart

@@ -7,3 +7,4 @@
 export "protocol_interfaces/protocol_client.dart";
 export "protocol_interfaces/protocol_client_factory.dart";
 export "protocol_interfaces/protocol_server.dart";
+export "protocol_interfaces/protocol_subscription.dart";

lib/src/core/protocol_interfaces/protocol_subscription.dart

@@ -0,0 +1,30 @@
+// Copyright 2024 Contributors to the Eclipse Foundation. 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 "package:meta/meta.dart";
+
+import "../scripting_api.dart";
+
+/// Base class for implementations of the [Subscription] interface.
+abstract base class ProtocolSubscription implements Subscription {
+  /// Instantiates a new [ProtocolSubscription].
+  ///
+  /// The [_complete] callback will be called when the [ProtocolSubscription]
+  /// has been [stop]ped (either internally or externally).
+  ProtocolSubscription(this._complete);
+
+  final void Function() _complete;
+
+  @override
+  @mustCallSuper
+  Future<void> stop({
+    int? formIndex,
+    Map<String, Object>? uriVariables,
+    Object? data,
+  }) async {
+    _complete();
+  }
+}