Support for pinned messages (#49)

The first draft to support pinned messages. Fixes #39.

Co-authored-by: Henry-Hiles <henry@henryhiles.com>
Reviewed-on: Nexus/nexus#49

lib/models/requests/set_state_request.dart

@@ -0,0 +1,23 @@
+import "package:freezed_annotation/freezed_annotation.dart";
+import "package:nexus/models/content/content.dart";
+import "package:nexus/models/ms_duration.dart";
+part "set_state_request.freezed.dart";
+part "set_state_request.g.dart";
+
+@freezed
+abstract class SetStateRequest with _$SetStateRequest {
+  const factory SetStateRequest({
+    required String roomId,
+    required String type,
+    required String stateKey,
+    required Content content,
+
+    @JsonKey(name: "delay_ms", includeIfNull: false)
+    @MSDuration()
+    @Default(null)
+    Duration? delay,
+  }) = _SetStateRequest;
+
+  factory SetStateRequest.fromJson(Map<String, Object?> json) =>
+      _$SetStateRequestFromJson(json);
+}