feat: Add validator that leverages symfony/validation constraints. #82

Job	Run time
CI / Generate matrix	8s
CI / Context	1s
CI / Check governance	2s
CI / 8.1 prf-low mysql57 phpunit silverstripe-composable-validators	1m 41s
CI / 8.2 mariadb phpunit silverstripe-composable-validators	2m 20s
CI / 8.3 mysql80 phpunit silverstripe-composable-validators	2m 29s
CI / 8.1 mysql57 phplinting	1m 29s
CI / 8.1 mysql57 js	3m 45s
CI / 8.1 mysql57 endtoend admin	21m 41s
CI / 8.2 mysql57 endtoend cms	16m 5s
CI / Check unreleased changes	0s
CI / Patch release	0s
	49m 41s

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