Validate Pardot URL

Thanks fjgarlin. I like it, but I thought it'd be nice to have a test before committing it.

This patch changes the error message from "URL is invalid." to "The Pardot post url must be a valid URL.", which I think gives a bit more direction about what to do about the problem, as opposed to just saying that there is a problem. It also adds tests, but they don't work yet.

I'm getting this:

There was 1 error:

1) Drupal\Tests\webform_pardot\Functional\ConfigurationTest::testAddHandler
Drupal\Component\Plugin\Exception\PluginNotFoundException: Plugin ID 'pardot_submission' was not found.

/app/web/core/lib/Drupal/Core/Plugin/DefaultLazyPluginCollection.php:79
/app/web/modules/contrib/webform/src/Plugin/WebformHandlerPluginCollection.php:35
/app/web/core/lib/Drupal/Component/Plugin/LazyPluginCollection.php:80
/app/web/modules/contrib/webform/src/Entity/Webform.php:2538
/app/web/modules/contrib/webform_pardot/tests/src/Functional/ConfigurationTest.php:82

which is referring to the getHandler part of

    /** @var \Drupal\webform\Entity\Webform $webform */
    $webform = Webform::load($webformId);
    $handler = $webform->getHandler($handlerPluginId);
    $configuration = $handler->getConfiguration();
    self::assertTrue(isset($configuration['pardot_url']) && $configuration['pardot_url'] === 'https://example.com/', 'Whitespace is trimmed from the Pardot URL.');

I'm not sure what its problem is. Maybe the plugins are discovered and cached prior to the new plugin getting installed?

Maybe we should just commit it without that assertion, but it'd be annoying to have all that and not include an assertion that the configuration value is transformed correctly.

The tests failed for a different reason than what I described in #3: the name of a webform configuration doesn't match its ID. Not sure why that didn't happen to me locally, but this should fix, and leave us with the issue described by #3.

Awesome, thanks a lot fjgarlin.