Modify(backend/pom.xml): add hibernate-validator-annotation-processor dependency.
Refactor(ValidationErrorInterceptor): rename to ValidationInterceptor.
Modify(RegistrationDetailsDTO): add validation annotations to username and password fields.
Add(ValidatorProvider).
Add(ValidationViolationsInterceptor).

Add(RequestParameterValidator).
Add(RequestParameterValidatorImpl).

Modify(UserRegistrationService): switch to using a RequestParameterValidator for validating the registration details.
Modify(RegistrationDetailsDTO): move validation annotations to getters.

Refactor(ValidationInterceptor): rename to RegistrationErrorInterceptor.

Modify(backend/pom.xml): add missing dependency for validation-api.