add support for jumping to out-of-context events (#95)

Fixes #84.

Reviewed-on: #95

lib/models/requests/paginate_manual.dart

@@ -0,0 +1,22 @@
+import "package:freezed_annotation/freezed_annotation.dart";
+import "package:nexus/models/direction.dart";
+
+part "paginate_manual.freezed.dart";
+part "paginate_manual.g.dart";
+
+@Freezed(toJson: false, fromJson: false)
+@JsonSerializable()
+class const PaginateManualRequest({
+  required final String roomId,
+  // Root event ID of a thread to paginate
+  final String? threadRoot,
+  // Can be null for starting pagination of a thread
+  final String? since,
+  required final Direction direction,
+  final int limit = 20,
+}) with _$PaginateManualRequest {
+  Map<String, Object?> toJson() => _$PaginateManualRequestToJson(this);
+
+  factory PaginateManualRequest.fromJson(Map<String, Object?> json) =>
+      _$PaginateManualRequestFromJson(json);
+}