Add DTO, exceptions, Flyway config, JPA file and test that allows user registration.

scripts/register

@@ -0,0 +1,5 @@
+#!/bin/bash
+
+curl -s -X POST http://localhost:8080/auth/register \
+  -H "Content-Type: application/json" \
+  -d '{"username": "starkey", "password": "password"}'