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.cpp

@@ -0,0 +1,20 @@
+#include "MSAInteractive.h"
+
+MSAInteractive::MSAInteractive(AccountData* data, QObject* parent) : AuthContext(data, parent) {}
+
+void MSAInteractive::executeTask() {
+    m_requestsDone = 0;
+    m_xboxProfileSucceeded = false;
+    m_mcAuthSucceeded = false;
+
+    initMSA();
+
+    QVariantMap extraOpts;
+    extraOpts["prompt"] = "select_account";
+    m_oauth2->setExtraRequestParams(extraOpts);
+
+    beginActivity(Katabasis::Activity::LoggingIn);
+    m_oauth2->unlink();
+    *m_data = AccountData();
+    m_oauth2->link();
+}