feat: Add validator that leverages symfony/validation constraints. #75

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 13s
CI / 8.2 mariadb phpunit silverstripe-composable-validators	2m 58s
CI / 8.3 mysql80 phpunit silverstripe-composable-validators	2m 33s
CI / 8.1 mysql57 phplinting	1m 57s
CI / 8.1 mysql57 js	4m 35s
CI / 8.1 mysql57 endtoend admin	22m 38s
CI / 8.2 mysql57 endtoend cms	16m 40s
CI / Check unreleased changes	0s
CI / Patch release	0s
	53m 46s

Provide feedback