refactor(core): add GitHub to RegisteredSystemns

src/main/java/de/hftstuttgart/unifiedticketing/core/RegisteredSystems.java

 package de.hftstuttgart.unifiedticketing.core;
 
+import de.hftstuttgart.unifiedticketing.systems.github.GithubTicketSystemBuilder;
 import de.hftstuttgart.unifiedticketing.systems.gitlab.GitlabTicketSystemBuilder;
 
 import java.util.logging.Level;
@@ -26,6 +27,14 @@ public class RegisteredSystems
         return instance;
     }
 
+    /**
+     * Starts the builder mechanism for a GitHub connection
+     */
+    public GithubTicketSystemBuilder github()
+    {
+        return new GithubTicketSystemBuilder();
+    }
+
     /**
      * Starts the builder mechanism for a GitLab connection
      */