GH-3392 dirty initial MSA support that shares logic with Mojang flows

Both act as the first step of AuthContext.

launcher/minecraft/auth/flows/MSAInteractive.h

@@ -0,0 +1,10 @@
+#pragma once
+#include "AuthContext.h"
+
+class MSAInteractive : public AuthContext
+{
+    Q_OBJECT
+public:
+    explicit MSAInteractive(AccountData * data, QObject *parent = 0);
+    void executeTask() override;
+};