[Validator] The default option name can now be omitted when defining constraints as annotations

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

When a constraint has a default option, but other options should be set as well, the name of the default option had to be set explicitly when defining the constraint as annotation:

```php
/** @GreaterThan(value=2, message="Should be greater than 2") */
private $value;
```

After this PR, it is possible to omit the name of the default option:

```php
/** @GreaterThan(2, message="Should be greater than 2") */
private $value;
```