feat: Add validator that leverages symfony/validation constraints. #76

Job	Run time
CI / Context	1s
CI / Generate matrix	10s
CI / Check governance	1s
CI / 8.1 prf-low mysql57 phpunit silverstripe-composable-validators	2m 37s
CI / 8.2 mariadb phpunit silverstripe-composable-validators	2m 45s
CI / 8.3 mysql80 phpunit silverstripe-composable-validators	3m 5s
CI / 8.1 mysql57 phplinting	1m 52s
CI / 8.1 mysql57 js	4m 56s
CI / 8.1 mysql57 endtoend admin	22m 20s
CI / 8.2 mysql57 endtoend cms	16m 43s
CI / Check unreleased changes	0s
CI / Patch release	0s
	54m 30s

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