Abstract some of the logic in Entry::getSupportedSites

[ryansupercool]

When implementing a custom Element that supports propagation we find ourselves duplicating the logic within Entry::getSupportedSites. We only typically just follow what Craft does here in terms of the supported methods. Craft Commerce does the same thing and duplicates the logic in Entry::getSupportedSites. I assume other developers also typically follow this when supporting propagation.

Could some of this logic be abstracted into a helper? Maybe something like the following.

public function getSupportedSites(): array
{
    // Developer does their normal logic to determine the method to use and get the site settings.
    if (!isset($this->typeId)) {
        throw new InvalidConfigException('`typeId` must be set on the element.');
    }

    $type = $this->getType();

    // This is provided by Craft
    return $this->internalSupportedSitesHelper($type->propagationMethod, $type->getSiteSettings());
}

The helper would then look like this and take two arguments, the propagation method and an array of SiteSettings models. SiteSettings being a new base class that has siteId and enabledByDefault properties.

/**
* @param PropagationMethod $propagationMethod
* @param SiteSettings[] $siteSettings
*/
public function internalSupportedSitesHelper(PropagationMethod $propagationMethod, array $siteSettings)