Constraints hook into the default validations, new constraints should derive from BaseConstraint.
This commit contains the code needed to add constraints to our model and a unittest for the UniqueConstraint option.

Add the following to a model field, to force uniqueness for a combination of keys:

<Constraints>
  <check001>                                                 <!--validation name, unique within this field-->
    <message>number should be unique</message>               <!--the message to output on validation failure -->
    <type>OPNsense\Base\Constraints\UniqueConstraint</type>  <!--the class to construct, derived from OPNsense\Base\Constraints\BaseConstraint -->
    <addFields>                                              <!--optional additional fields for the equation, defined in UniqueConstraint -->
       <field1>optfield</field1>
    </addFields>
  </check001>
</Constraints>