A 'virtual properties' attribute

[gwillz]

We already have a very handy UpdateVirtualTrait that lets us apply functions whenever updating data via constructor or update(). This lets us normalise or parse complex data. However, it's a little cumbersome.

This lets one declare an attribute for properties that have a respective method that applies these normalisations, instead of defining them in the virtual() helper.

There are two core attributes:

• VirtualProperty - this defines a method to call when updating this property.
• VirtualObject - this wraps a value (or array of values) in the target object.

Such as:

class MyModel extends DataObject
{
   use UpdateTrait;
   use AttributesVirtualTrait;

   #[VirtualProperty('setComplex')]
   public $complex;

   #[VirtualObject(OtherModel::class)]
   public $object;
   
   public function setComplex($value)
   {
      this->complex = new Complex($value);
   }
}

This PR also includes a helper trait (VirtualHelperTrait) that creates helpers to configure objects on update().

Draft because:

• no tests
• I want to rename VirtualHelperTrait to something more obvious? Or just bundle it with the existing UpdateVirtualTrait?
• I figure the post-modification style applyVirtual() isn't appropriate for strict typings in PHP 8+. This is fixed in the feat/hooks branch.