Plan for field validation 3.0.0

Problem/Motivation

1, add support for Constraint API , see issue #2541228: FieldConfigBase::setPropertyConstraints() and addPropertyConstraints() are broken → ， I plan support it in 8.x-1.x， but Drupal's constraint API is not stable enough when Drupal 8.0 released.

2, allow user add validation at field config page, maybe in a separate module.

3，Add support for Symfony Constraints.

4, add some basic tests for new added constraint rule.