Fix the problem of query Birthdays in ORM

- [x] Seggregate interfaces
- [x] Make DQL Functions works with `MySQL`, `PostgreSQL` and `SQLite`
- [x] Make tests with **2** implementations of `AbstractPerson`: `User` and `Employee`
- [x] Make `BirthdayRepository` handle all subclasses of `AbstractPerson`
- [x] Make `BirthdayRepository` provides only birthdays **of the day**, **for now**
- [x] Init `BirthdayHelper` development: Inject a `BirthdayProvider`.

The next step is to make `BirthdayHelper` consumes and render data provided by `BirthdayProvider`.