refactor(core/TicketSystem): move instance field configuration to rest of instance definition

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

@@ -4,6 +4,7 @@ import de.hftstuttgart.unifiedticketing.exceptions.AssertionException;
 import de.hftstuttgart.unifiedticketing.systems.gitlab.GitlabTicketSystem;
 
 import java.util.HashMap;
+import java.util.Map;
 import java.util.logging.Level;
 import java.util.logging.Logger;
 import java.util.regex.Matcher;
@@ -22,8 +23,6 @@ public abstract class TicketSystem<T extends Ticket, TS extends TicketSystem, TB
 {
     private static Logger logger = Logging.getLogger(TicketSystem.class.getName());
 
-    protected HashMap<String, String> configuration = new HashMap<>();
-
     /**
      * Global configuration options for ticket system instances, settable with the method config
      */
@@ -90,6 +89,8 @@ public abstract class TicketSystem<T extends Ticket, TS extends TicketSystem, TB
     public String password;
     public String username;
 
+    protected Map<String, String> configuration = new HashMap<>();
+
     /**
      * sets a value configuration
      * @param option configuration to set