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/tests/src/Kernel/RelatedPostsTest.php

@@ -0,0 +1,38 @@
+<?php
+
+// phpcs:disable Drupal.Commenting.DocComment, Drupal.NamingConventions.ValidFunctionName
+
+namespace Drupal\Tests\opdavies_blog\Kernel;
+
+use Drupal\opdavies_blog\Repository\RelatedPostsRepository;
+use Drupal\opdavies_blog_test\Factory\PostFactory;
+
+final class RelatedPostsTest extends PostTestBase {
+
+  private RelatedPostsRepository $relatedPostsRepository;
+
+  /** @test */
+  public function it_returns_related_posts(): void {
+    $postA = (new PostFactory())->setTitle('Post A')
+      ->withTags(['Drupal 8'])
+      ->create();
+    $postA->save();
+
+    $postB = (new PostFactory())->setTitle('Post B')
+      ->withTags(['Drupal 8'])
+      ->create();
+    $postB->save();
+
+    $relatedPosts = $this->relatedPostsRepository->getFor($postA);
+
+    $this->assertCount(1, $relatedPosts);
+    $this->assertSame('Post B', $relatedPosts->first()->label());
+  }
+
+  protected function setUp() {
+    parent::setUp();
+
+    $this->relatedPostsRepository = $this->container->get(RelatedPostsRepository::class);
+  }
+
+}