Extract a PostPusher service to push the post

Extract a `PostPusher` service with an IFTTT implementation. That means
if this needs to change to use a different service in the future, the
IFTTT implementation can remain unchanged and a new implementation can
be added with its own form params etc.

This also makes this functionality easier to test as we can add a
`NullPostPusher` implementation for testing.

References #332

web/modules/custom/blog/src/Service/PostPusher/HttpPostPusher.php

@@ -0,0 +1,17 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\opdavies_blog\Service\PostPusher;
+
+use GuzzleHttp\ClientInterface;
+
+abstract class HttpPostPusher implements PostPusher {
+
+  protected ClientInterface $client;
+
+  public function __construct(ClientInterface $client) {
+    $this->client = $client;
+  }
+
+}

web/modules/custom/blog/src/Service/PostPusher/IftttPostPusher.php

@@ -0,0 +1,39 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\opdavies_blog\Service\PostPusher;
+
+use Drupal\Core\Config\ConfigFactoryInterface;
+use Drupal\opdavies_blog\Entity\Node\Post;
+use GuzzleHttp\ClientInterface;
+use Webmozart\Assert\Assert;
+
+final class IftttPostPusher extends HttpPostPusher {
+
+  private ConfigFactoryInterface $configFactory;
+
+  public function __construct(
+    ClientInterface $client,
+    ConfigFactoryInterface $configFactory
+  ) {
+    $this->configFactory = $configFactory;
+
+    parent::__construct($client);
+  }
+
+  public function push(Post $post): void {
+    $url = $this->configFactory
+      ->get('opdavies_blog.settings')
+      ->get('post_tweet_webhook_url');
+
+    Assert::notNull($url, 'Cannot push the post if there is no URL.');
+
+    $this->client->post($url, [
+      'form_params' => [
+        'value1' => $post->toTweet(),
+      ],
+    ]);
+  }
+
+}

web/modules/custom/blog/src/Service/PostPusher/PostPusher.php

@@ -0,0 +1,13 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\opdavies_blog\Service\PostPusher;
+
+use Drupal\opdavies_blog\Entity\Node\Post;
+
+interface PostPusher {
+
+  public function push(Post $post): void;
+
+}