Push Notifications (#66)

Fixes #35.

Things to do:
- [x] Register with local distributor
- [x] Register with homeserver
- [x] Recieve messages
- [x] Show notifications
- [ ] Notifications page (could come later)

Reviewed-on: Nexus/nexus#66

lib/models/requests/deregister_pusher.dart

@@ -0,0 +1,13 @@
+import "package:freezed_annotation/freezed_annotation.dart";
+
+part "deregister_pusher.freezed.dart";
+part "deregister_pusher.g.dart";
+
+@Freezed(toJson: false, fromJson: false)
+@JsonSerializable()
+class const DeregisterPusherRequest({
+  required final String appId,
+  @JsonKey(name: "pushkey") required final String pushKey,
+}) with _$DeregisterPusherRequest {
+  Map<String, Object?> toJson() => _$DeregisterPusherRequestToJson(this);
+}

lib/models/requests/register_pusher.dart

@@ -0,0 +1,56 @@
+import "package:freezed_annotation/freezed_annotation.dart";
+
+part "register_pusher.freezed.dart";
+part "register_pusher.g.dart";
+
+@Freezed(toJson: false, fromJson: false)
+@JsonSerializable()
+class const RegisterPusherRequest({
+  required final String appDisplayName,
+  required final String appId,
+  final bool append = false,
+  required final PusherData data,
+  required final String deviceDisplayName,
+  required final PusherKind kind,
+  required final String lang,
+
+  /// TODO: What does this do?
+  final String? profileTag,
+
+  @JsonKey(name: "pushkey") required final String pushKey,
+}) with _$RegisterPusherRequest {
+  Map<String, Object?> toJson() => _$RegisterPusherRequestToJson(this);
+}
+
+@freezed
+sealed class PusherData with _$PusherData {
+  const factory PusherData.http({
+    required Uri url,
+    @Default(PushFormat.eventIdOnly) PushFormat format,
+  }) = HttpPusherData;
+
+  const factory PusherData.webPush({
+    required Uri url,
+    @Default(PushFormat.eventIdOnly) PushFormat format,
+
+    /// `data.auth`: RFC8291 authentication secret.
+    required String auth,
+  }) = WebPushPusherData;
+
+  factory PusherData.fromJson(Map<String, Object?> json) =>
+      _$PusherDataFromJson(json);
+}
+
+@JsonEnum(fieldRename: .snake)
+enum PushFormat {
+  @JsonValue(null)
+  all,
+  eventIdOnly,
+}
+
+enum PusherKind {
+  http,
+  email,
+  @JsonValue("org.matrix.msc4174.webpush")
+  webPush,
+}