Implement /marker command (#2360)

This command works the same as it does on Twitch web chat - it creates a streamer marker at the current timestamp with an optional description

src/providers/twitch/api/Helix.hpp

@@ -165,12 +165,33 @@ struct HelixChannel {
     }
 };
 
+struct HelixStreamMarker {
+    QString createdAt;
+    QString description;
+    QString id;
+    int positionSeconds;
+
+    explicit HelixStreamMarker(QJsonObject jsonObject)
+        : createdAt(jsonObject.value("created_at").toString())
+        , description(jsonObject.value("description").toString())
+        , id(jsonObject.value("id").toString())
+        , positionSeconds(jsonObject.value("position_seconds").toInt())
+    {
+    }
+};
+
 enum class HelixClipError {
     Unknown,
     ClipsDisabled,
     UserNotAuthenticated,
 };
 
+enum class HelixStreamMarkerError {
+    Unknown,
+    UserNotAuthorized,
+    UserNotAuthenticated,
+};
+
 class Helix final : boost::noncopyable
 {
 public:
@@ -242,6 +263,12 @@ public:
                     ResultCallback<HelixChannel> successCallback,
                     HelixFailureCallback failureCallback);
 
+    // https://dev.twitch.tv/docs/api/reference/#create-stream-marker
+    void createStreamMarker(
+        QString broadcasterId, QString description,
+        ResultCallback<HelixStreamMarker> successCallback,
+        std::function<void(HelixStreamMarkerError)> failureCallback);
+
     void update(QString clientId, QString oauthToken);
 
     static void initialize();