Allow StaticPublishingTrigger to be added in DataExtensions

[tiller1010]

Description

I would like to add StaticPublishingTrigger to a DataObject through a DataExtension, but StaticPublishingTrigger checks if the original DataObject implements StaticPublishingTrigger.

// I want the Resource DataObject to queue an update to its own URL when changed.
class ResourceExtension extends DataExtension implements StaticallyPublishable, StaticPublishingTrigger
{
  public function urlsToCache()
  {
    return [$this->owner->getDetailPageAbsoluteLink() => 0];
  }

  public function objectsToUpdate($context)
  {
    return $this->owner;
  }
}

What if there was a loop over extensions that checked for this interface?

$extensionImplementsInterface = false;
foreach ($siteTree->get_extensions() as $extension) {
    if (new $extension instanceof StaticPublishingTrigger) {
        $extensionImplementsInterface = true;
        break;
    }
}

// The page does not include the required extension, and it doesn't implement a Trigger
if (!$siteTree->hasExtension(PublishableSiteTree::class) && !($siteTree instanceof StaticPublishingTrigger) && !$extensionImplementsInterface) {
    return;
}

Additional context or points of discussion

This is how I have the extensions applied

---
Name: add_static_publishing_methods_to_resource
---
VendorName\ResourcePackage\Resources\Resource:
  extensions:
    - App\Resources\ResourceExtension

---
Name: update_resource_detail_on_change
After: add_static_publishing_methods_to_resource
---
VendorName\ResourcePackage\Resources\Resource:
  extensions:
    - SilverStripe\StaticPublishQueue\Extension\Engine\SiteTreePublishingEngine

Validations

• You intend to implement the feature yourself
• You have read the contributing guide
• You strongly believe this feature should be in core, rather than being its own community module
• You have checked for existing issues or pull requests related to this feature (and didn't find any)

[GuySartorelli]

What if there was a loop over extensions that checked for this interface?

Not neecssary - you can do $record->hasExtension(StaticPublishingTrigger::class) which will check for extensions that implement that interface.

Assuming implementing the interface on an extension class actually works, I see no reason not to allow this. Feel free to submit a PR including unit tests that validate triggers work as expected when provided through an extension.