Add stub for Compound::getConstraints()

[derrabus]

This PR fixes a error people get when implementing compound constraints:

Method App\Validator\Constraints\MyConstraint::getConstraints() has parameter $options with no value type specified in iterable type array.

Symfony does not document the structure of the $options array, which can be an array of anything.

[ondrejmirtes]

Thank you! :)

[VincentLanglet]

Symfony does not document the structure of the $options array, which can be an array of anything.

Looking at the usage, I think it should be array<string, mixed> rather than array<mixed>

I created symfony/symfony#54574 to validate the point @derrabus.

[derrabus]

Thank you for opening the PR upstream. I've left a comment.