feat: Add validator that leverages symfony/validation constraints. #80

Job	Run time
CI / Context	1s
CI / Generate matrix	8s
CI / Check governance	1s
CI / 8.1 prf-low mysql57 phpunit silverstripe-composable-validators	1m 39s
CI / 8.2 mariadb phpunit silverstripe-composable-validators	2m 19s
CI / 8.3 mysql80 phpunit silverstripe-composable-validators	2m 16s
CI / 8.1 mysql57 phplinting	1m 25s
CI / 8.1 mysql57 js	3m 55s
CI / 8.1 mysql57 endtoend admin	21m 58s
CI / 8.2 mysql57 endtoend cms	16m 4s
CI / Check unreleased changes	0s
CI / Patch release	0s
	49m 46s

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