[Validator] Made it possible to store the cause of a constraint violation

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | TODO

This change makes it possible to store the cause of a violation (e.g. an exception). This way it is possible to follow the trace of violations caused by exceptions up to their root.

```php
try {
    // ...
} catch (Exception $e) {
    $this->context->buildViolation('Error!')
        ->setCause($e)
        ->addViolation();
}
```

This is one step to solve #5607.
weird line here ;)