Introduce the StructuredFieldAccess interface

[nyamsprod]

The interface allow adding custom structured field. If a class implement the interface it will be able to be added to each DataType with ease.

For instance you will be able to do the following:

$myObject = new class () implements StructuredFieldAccess {
    public function toStructuredField(): StructuredField
    {
        return Item::new(Token::fromString('babayaga'));
    }
};


$instance = InnerList::fromAssociative([$myObject, 'a', true], ['a' => true]);
echo $instance->toHttpValue();
// prints '(babayaga "a" ?1);a' 

On StructuredField creation the toStructuredField method is called and its return value is
used and store in place of the $myObject instance.