Return related posts for a specified blog post

Add the initial code for specifying a blog post and returning related
posts. This includes adding a repository for related posts, and adding
it as a service within the `opdavies_blog` module.

References #3

web/modules/custom/blog/src/Repository/RelatedPostsRepository.php

@@ -0,0 +1,32 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\opdavies_blog\Repository;
+
+use Drupal\Core\Entity\EntityStorageInterface;
+use Drupal\Core\Entity\EntityTypeManagerInterface;
+use Drupal\opdavies_blog\Entity\Node\Post;
+use Illuminate\Support\Collection;
+
+final class RelatedPostsRepository {
+
+  private EntityStorageInterface $nodeStorage;
+
+  public function __construct(
+    EntityTypeManagerInterface $entityTypeManager
+  ) {
+    $this->nodeStorage = $entityTypeManager->getStorage('node');
+  }
+
+  public function getFor(Post $post): Collection {
+    $posts = $this->nodeStorage->loadByProperties([
+      'type' => 'post',
+    ]);
+
+    return (new Collection($posts))
+      ->filter(fn(Post $relatedPost) => $relatedPost->id() != $post->id())
+      ->values();
+  }
+
+}