Password validation

It's time to improve the validation for the password.

In the Core module, in the Validation section, we used some of the decorators from the Class Validator package to validate the User's fields. However, the password field was left with the simplest validation. In this section, we'll improve it.

We'll do so using a Regular Expression (regex) to have a more flexible validation. However, it would become an untidy solution if we just left it in the DTO itself. Due to this reason, we'll also implement a Custom Validator to encapsulate this validation.