Combine cached classes into a UNION to cut down on extravagant queries

[sabina-talipova]

Description

Investigate possible improvements for QueryCachingMiddleware::getCachedResponse by using DataQuery::union method to reduce additional queries.

See:

silverstripe-graphql/src/Middleware/QueryCachingMiddleware.php

Lines 112 to 121 in 4c66b57

	// On cache success validate against cached classes
	foreach ($cached['classes'] as $class) {
	// Note: Could combine these clases into a UNION to cut down on extravagant queries
	// Todo: We can get last-deleted/modified as well for versioned records
	$lastEditedDate = DataObject::get($class)->max('LastEdited');
	if (strtotime($lastEditedDate ?? '') > strtotime($cached['date'] ?? '')) {
	// class modified, fail validation of cache
	return null;
	}
	}