NEW Wire up symfony/validator

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

https://github.com/silverstripe/silverstripe-linkfield/issues/17

Pull request checklist

[x] The target branch is correct
- See picking the right version
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[x] The commit messages follow our commit message guidelines
[x] The PR follows our contribution guidelines
[x] Code changes follow our coding conventions
[x] This change is covered with tests (or tests aren't necessary for this change)
[x] Any relevant User Help/Developer documentation is updated; for impactful changes, information is added to the changelog for the intended release
[x] CI is green

silverstripe / silverstripe-framework