[Validator][WIP] Support validation for German banking

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        | symfony/symfony-docs#3976

- [ ] submit changes to the documentation

Hi
I'm the author of malkusch/bav, a validation library for German bank accounts. This pull request is the integration of bav into Symfony's validation. You get those new constraints:
* `@Bic`
* `@Konto`
* `@Blz`

Enjoy!
we never put such guards in Symfony constraints, relying on the validator logic choosing the ConstraintValidator