[WIP][WCM][constraints] Support validation for German banking

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#11231)
| Applies to    | master
| Fixed tickets | N/A

- [ ] document `@Konto`
- [ ] document `@Blz`

This documents the new constraints 
* `@Blz`
* `@Bic`
* `@Konto`