Add LoginCredentials class for json parsing

src/main/java/com/example/plantsforyou/appuser/LoginCredentials.java

@@ -0,0 +1,14 @@
+package com.example.plantsforyou.appuser;
+
+public class LoginCredentials {
+    private String username;
+    private String password;
+
+    public String getUsername() {
+        return username;
+    }
+
+    public String getPassword() {
+        return password;
+    }
+}