First stubs to register PreferenceConverters on the UserStore

java/com.sap.sse.security/src/com/sap/sse/security/PreferenceConverter.java

@@ -0,0 +1,7 @@
+package com.sap.sse.security;
+
+public interface PreferenceConverter<PREF> {
+    String toString(PREF preference);
+
+    PREF toPreference(String stringPreference);
+}

java/com.sap.sse.security/src/com/sap/sse/security/UserStore.java

@@ -106,4 +106,6 @@ public interface UserStore extends Named {
      * {@code username} no access token has previously been {@link #setAccessToken(String, String) set}.
      */
     String getAccessToken(String username);
+    
+//    void registerPreferenceConverter(String preferenceKey, PreferenceConverter<?> converter);
 }