added UserHelper interface to allow dependency injection

Signed-off-by: Stephan Richter <s.richter@srsoftware.de>

core/src/main/java/de/srsoftware/umbrella/core/api/UserHelper.java

@@ -0,0 +1,11 @@
+package de.srsoftware.umbrella.core.api;
+
+import com.sun.net.httpserver.HttpExchange;
+import de.srsoftware.umbrella.core.UmbrellaException;
+import de.srsoftware.umbrella.core.model.UmbrellaUser;
+
+import java.util.Optional;
+
+public interface UserHelper {
+	Optional<UmbrellaUser> loadUser(HttpExchange ex) throws UmbrellaException;
+}