Add user registration endpoint and HDFS directory creation

- Implemented a new registration endpoint in AuthController to handle user sign-ups.
- Integrated AuthenticationService to save the user and create a corresponding HDFS directory for the user upon registration.
- Updated User entity to return the username instead of email in getUsername method.

src/main/java/com/skycrate/backend/skycrateBackend/entity/User.java

@@ -56,7 +56,7 @@ public class User implements UserDetails {
 
     @Override
     public String getUsername() {
-        return email; // or return username if that's your login key
+        return username; // or return username if that's your login key
     }
 
     @Override