feat: Add validator that leverages symfony/validation constraints. #77

Job	Run time
CI / Generate matrix	6s
CI / Context	1s
CI / Check governance	1s
CI / 8.1 prf-low mysql57 phpunit silverstripe-composable-validators	1m 56s
CI / 8.2 mariadb phpunit silverstripe-composable-validators	2m 7s
CI / 8.3 mysql80 phpunit silverstripe-composable-validators	2m 25s
CI / 8.1 mysql57 phplinting	1m 40s
CI / 8.1 mysql57 js	4m 42s
CI / 8.1 mysql57 endtoend admin	21m 55s
CI / 8.2 mysql57 endtoend cms	16m 19s
CI / Check unreleased changes	0s
CI / Patch release	0s
	51m 12s

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