NEW Wire up symfony/validator

[GuySartorelli]

Description

Wires up symfony/validator so we can use it for robust and potentially complex validation logic.

Usage is:

use SilverStripe\Core\Validation\ConstraintValidator;

/**
 * @var \Symfony\Component\Validator\Constraint $constraint
 * @var \SilverStripe\ORM\ValidationResult $result
 */
$result = ConstraintValidator::validate($valueToValidate, $constraint);

e.g. for testing if a URL is valid:

use SilverStripe\Core\Validation\ConstraintValidator;
use Symfony\Component\Validator\Constraints\Url;

$isValid = ConstraintValidator::validate($url, new Url())->isValid();

Most of the constraints in https://symfony.com/doc/current/reference/constraints.html are explicitly supported, except for the following:

Any which require symfony/expression-language:

• Symfony\Component\Validator\Constraints\Expression
• Symfony\Component\Validator\Constraints\ExpressionSyntax
• Symfony\Component\Validator\Constraints\When

Any which require symfony/http-client:

• Symfony\Component\Validator\Constraints\NotCompromisedPassword

Any which require symfony/intl:

• Symfony\Component\Validator\Constraints\Bic
• Symfony\Component\Validator\Constraints\Country
• Symfony\Component\Validator\Constraints\Currency
• Symfony\Component\Validator\Constraints\Language
• Symfony\Component\Validator\Constraints\Locale

Any which require symfony/doctrine or anything which assumes we're using symfony/validate in the context of setting constraints on metadata objects:

• Symfony\Component\Validator\Constraints\Cascade
• Symfony\Component\Validator\Constraints\Traverse
• Symfony\Component\Validator\Constraints\Valid

With exception of those last three (which I've mentioned in the docs are explicitly not supported, because it relies on extra wiring up of things that we just aren't gonna do), I'd expect the others to all work - I just haven't included tests for them because we don't have those dependencies.

Issues

• Validation of standard link types silverstripe-linkfield#17

Pull request checklist

• The target branch is correct
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• The commit messages follow our commit message guidelines
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• This change is covered with tests (or tests aren't necessary for this change)
• Any relevant User Help/Developer documentation is updated; for impactful changes, information is added to the changelog for the intended release
• CI is green

[emteknetnz]

Add a unit test(s) for passing in an array of contraints

Add a unit test for passing in optional field param

[GuySartorelli]

Add a unit test(s) for passing in an array of contraints

Done

Add a unit test for passing in optional field param

Done

Also added an exception (and test for it) when passing in an empty array.