BE: Allow registered users to login

1. Where to store user data (login, password, email, name, etc.)
2. How to store (password to be salted and hashed)
3. Session/JWT token?
4. Please investigate options available in Spring Security
5. Implement the solution.

Please note: on this stage users to be created in DB manually. Refer to https://www.baeldung.com/security-spring