GH-4071 Heavily refactor and rearchitect account system

This makes the account system much more modular
and makes it treat errors as something recoverable,
unless they come directly from the MSA refresh token
becoming invalid.

launcher/minecraft/auth/steps/MigrationEligibilityStep.cpp

@@ -0,0 +1,45 @@
+#include "MigrationEligibilityStep.h"
+
+#include <QNetworkRequest>
+
+#include "minecraft/auth/AuthRequest.h"
+#include "minecraft/auth/Parsers.h"
+
+MigrationEligibilityStep::MigrationEligibilityStep(AccountData* data) : AuthStep(data) {
+
+}
+
+MigrationEligibilityStep::~MigrationEligibilityStep() noexcept = default;
+
+QString MigrationEligibilityStep::describe() {
+    return tr("Checking for migration eligibility.");
+}
+
+void MigrationEligibilityStep::perform() {
+    auto url = QUrl("https://api.minecraftservices.com/rollout/v1/msamigration");
+    QNetworkRequest request = QNetworkRequest(url);
+    request.setHeader(QNetworkRequest::ContentTypeHeader, "application/json");
+    request.setRawHeader("Authorization", QString("Bearer %1").arg(m_data->yggdrasilToken.token).toUtf8());
+
+    AuthRequest *requestor = new AuthRequest(this);
+    connect(requestor, &AuthRequest::finished, this, &MigrationEligibilityStep::onRequestDone);
+    requestor->get(request);
+}
+
+void MigrationEligibilityStep::rehydrate() {
+    // NOOP, for now. We only save bools and there's nothing to check.
+}
+
+void MigrationEligibilityStep::onRequestDone(
+    QNetworkReply::NetworkError error,
+    QByteArray data,
+    QList<QNetworkReply::RawHeaderPair> headers
+) {
+    auto requestor = qobject_cast<AuthRequest *>(QObject::sender());
+    requestor->deleteLater();
+
+    if (error == QNetworkReply::NoError) {
+        Parsers::parseRolloutResponse(data, m_data->canMigrateToMSA);
+    }
+    emit finished(AccountTaskState::STATE_WORKING, tr("Got migration flags"));
+}