Allow caching of draft webform pages

Please review the merge request I've just created, which use session cache context.

Let me know what do you think, thank you!

Webform paragraphs drafts are failing in the last pipeline:

1) Drupal\Tests\webform\Functional\Paragraphs\WebformParagraphsDraftsTest::testParagraphsDrafts
Behat\Mink\Exception\ExpectationException: The string "A partially-completed form was found. Please complete the remaining portions." was not found anywhere in the HTML response of the current page.

Checking why it fails.

I've noticed:

1. The session cache context is already added https://git.drupalcode.org/project/webform/-/blob/6.2.x/src/WebformSubmi... . So the current patch needs work as it is only removing the cache invalidation.
2. The test passes when the webform is added with source entity, but not when it is added without source entity.

Still checking for solutions.

Fixed the MR to only invalidate caches if the submission does not have source entity. Now tests should pass.

I've tested it and it works! Response header "Cache-Control:" goes from "must-revalidate, no-cache, private" to "max-age=86400, public". As in tested case, source entity is always used. Anyone that also wants to check this consider any other module or functionality using "\Drupal::service(('page_cache_kill_switch'))->trigger();" may affect. Any case, more opinions & review are preferred i think.

+1 to RTBC!