Allow caching of webforms referenced through WebformEntityViewBuilder

Among other things, this actually disables the render cache of any entity this is embedded into as it bubbles up all the way up to the node and then dynamic page cache. (It's worth pointing out that forms in core actually do the same right now for authenticated users, so this will only make a difference for anonymous users).

It's also interesting that \Drupal\webform\Plugin\Field\FieldFormatter\WebformEntityReferenceEntityFormatter::viewElements() calls $entity->getSubmissionForm() directly, so skips the view builder, which means this is *only* used for standard entity reference fields (Which we actually use in a paragraph type as the additional settings the webform field exposes isn't something we want to have there).

What field did you use for testing, a normal entity reference field or a webform field? As mentioned, the webform field formatter actually doesn't even use this and therefore doesn't have caching disabled.

Considering that, it seems pretty pretty safe to change?

For consistency, we should then also remove \Drupal\webform\Plugin\Block\WebformBlock::getCacheMaxAge().

> Page cache should be enabled

max-age does not affect the anonymous page cache: #2352009: [pp-3] Bubbling of elements' max-age to the page's headers and the page cache. So you will not be able to see a difference with or without that actually.

The only thing you can see to confirm that it is working would be that you can see cache entries for the affected block/node in the cache_render table for anonymous users (but not for authenticated ones, because the form itself is still not cacheable until #2578855: Form tokens are now rendered lazily, allow forms to opt in to be cacheable is fixed).

One possible option for that would be to make the form a lazy-builder component, which should actually already happen for blocks. I did that for the poll module because that is highly-personalized based on user/cookie. The result would be that the rest of the rendered node would still be cached and just the form would be then inserted later. With big pipe, you should be able to visually see that the form is inserted after the initial page load then. See \Drupal\poll\PollViewBuilder::view().

But I think this can be done in a separate issue.