Add support for Symfony IdenticalTo Constraint

test case:
add a EqualTo constraint to field_test, set value to "drupal", save it.

if populate field_test "drupal", no error message.

if populate field_test "123456", get an error message

Note：
the value option support token, when using "default" validate mode.

IdenticalTo use "==="
EqualTo use "=="
A little different.

Automatically closed - issue fixed for 2 weeks with no activity.