GH-4317 Detect forced migration state and show errors for it

launcher/minecraft/auth/steps/ForcedMigrationStep.h

@@ -0,0 +1,23 @@
+#pragma once
+#include <QObject>
+
+#include "QObjectPtr.h"
+#include "minecraft/auth/AuthStep.h"
+
+
+class ForcedMigrationStep : public AuthStep {
+    Q_OBJECT
+
+public:
+    explicit ForcedMigrationStep(AccountData *data);
+    virtual ~ForcedMigrationStep() noexcept;
+
+    void perform() override;
+    void rehydrate() override;
+
+    QString describe() override;
+
+private slots:
+    void onRequestDone(QNetworkReply::NetworkError, QByteArray, QList<QNetworkReply::RawHeaderPair>);
+};
+