feat: add initial experimental Twitch Eventsub support (#5837)

Co-authored-by: nerix <nerixdev@outlook.de>

mocks/include/mocks/EmptyApplication.hpp

@@ -271,6 +271,13 @@ public:
         return nullptr;
     }
 
+    eventsub::IController *getEventSub() override
+    {
+        assert(false && "EmptyApplication::getEventSub was called without "
+                        "being initialized");
+        return nullptr;
+    }
+
     QTemporaryDir settingsDir;
     Paths paths_;
     Args args_;

mocks/include/mocks/Helix.hpp

@@ -427,6 +427,15 @@ public:
          FailureCallback<QString> failureCallback),
         (override));
 
+    MOCK_METHOD(void, createEventSubSubscription,
+                (const eventsub::SubscriptionRequest &request,
+                 const QString &sessionID,
+                 ResultCallback<HelixCreateEventSubSubscriptionResponse>
+                     successCallback,
+                 (FailureCallback<HelixCreateEventSubSubscriptionError, QString>
+                      failureCallback)),
+                (override));
+
     MOCK_METHOD(void, update, (QString clientId, QString oauthToken),
                 (override));