feat: add `UserModel::getReturnType()` and RegisterController::getUserEntity() uses it

[christianberkman]

Description See #1170 Refactor RegisterController::getUserEntity to use UserModel::returnType instead of the hard coded new User(). This allows for easy use of a custom User Entity without needing to extend the RegisterController and add the routes.

Checklist:

• [x] Securely signed commits
• [x] Component(s) with PHPDoc blocks, only if necessary or adds value
• [ ] Unit testing, with >80% coverage
• [x] User guide updated
• [x] Conforms to style guide

[kenjis]

I sent #1196 Check, please.