TF-320 Add capability validator for Jmap request

lib/main/error/request_error.dart

@@ -0,0 +1,11 @@
+import 'package:equatable/equatable.dart';
+import 'package:jmap_dart_client/jmap/core/capability/capability_identifier.dart';
+
+class SessionMissingCapability extends Equatable implements Exception {
+  final Set<CapabilityIdentifier> capabilityIdentifiers;
+
+  SessionMissingCapability(this.capabilityIdentifiers);
+
+  @override
+  List<Object?> get props => [capabilityIdentifiers];
+}