[Bridge][Doctrine][Validator] allow UniqueEntity constraint to be set on external class

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Use case for this feature would be if you want to validate data on a DTO before actually mapping it to the Entity itself.