[Validator] Added missing error codes and turned codes into UUIDs

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

With the current implementation of error codes, checking which error occurred is unnecessarily complex:

```php
if ($violation->getConstraint() instanceof Length && Length::TOO_SHORT_ERROR === $violation->getCode()) {
    // ...
}
```

Also, the code is completely missing for some constraints. This is fixed now. By using UUIDs, the check is reduced to:

```php
if (Length::TOO_SHORT_ERROR === $violation->getCode()) {
    // ...
}
```

Also, APIs can simply output the error code and the name of the error without needing to point to the constraint as well.

Before:

```json
"errors": [
    {
        "code": "1",
        "name": "TOO_SHORT_ERROR",
        "message": "This value is too short. ...",
        "constraint": "Symfony\Component\Validator\Constraints\Length"
    }
]
```

After:

```json
"errors": [
    {
        "code": "9ff3fdc4-b214-49db-8718-39c315e33d45",
        "name": "TOO_SHORT_ERROR",
        "message": "This value is too short. ..."
    }
]
```

This makes it possible to implement a service on symfony.com which looks up error codes, e.g.

symfony.com/error?code=9ff3fdc4-b214-49db-8718-39c315e33d45

Such a URL could redirect directly to the documentation of the appropriate constraint.