feat: Add InvalidateElementCachesEvent that fires when element TagDependency caches are invalidated

[khalwat]

Description

This PR adds an InvalidateElementCachesEvent that is triggered by three methods in the Elements service:

• invalidateAllCaches()
• invalidateCachesForElementType()
• invalidateCachesForElement()

These methods are bottleneck methods that are called from a wide variety of places in the codebase whenever an element's caches need to be invalidated.

The event has a $tags property that is an array of TagDependency tag names that are being invalidated, so the listener can take an action when the caches are invalidated. Otherwise, there is no way to know when an Element's caches have been invalidated because the TagDependency information is stored appended to the cached data via whatever the current caching method is.

Throwing these events will greatly simplify implementing a variety of middleware that needs to know when an element's caches have been invalidated. Example usage:

        // Listen for invalidated element caches
        Event::on(
            Elements::class,
            Elements::EVENT_INVALIDATE_ELEMENT_CACHES,
            static function (InvalidateElementCachesEvent $event) {
                $tags = $event->tags;
                // Do something with the $tags
            }
        );

Plugins like Blitz, Upper, and others that deal with cache invalidation would benefit from this, rather than having to roll their own solution.

These events are intentionally agnostic; you'd layer something useful on top of them. A common use-case would be to use these events in conjunction with something like:

        if (Craft::$app->getRequest()->getIsSiteRequest()) {
            // Start collecting cache tags
            Event::on(
                View::class,
                View::EVENT_BEFORE_RENDER_PAGE_TEMPLATE,
                static function (TemplateEvent $event) {
                    Craft::$app->getElements()->startCollectingCacheTags();
                }
            );
            // Stop collecting cache tags
            Event::on(
                View::class,
                View::EVENT_AFTER_RENDER_PAGE_TEMPLATE,
                static function (TemplateEvent $event) {
                    $tags = Craft::$app->getElements()->stopCollectingCacheTags();

                }
            );
        }

...so that you can collect the element caches on a given page, and when those events fire, do whatever you need to do to invalidate them.

In my case, I will be using a custom db table to store the rendering page's URI in conjunction with a JSON column type that has the array of TagDependency tag names, so whenever a tag is invalidated, I can do a reverse lookup to find all of the affected URIs.

[brandonkelly]

4.2.0 is out now with that event ✨