Added simple way to resolve domains from entity manager when creating a short URL

module/Core/src/Domain/Resolver/DomainResolverInterface.php

@@ -0,0 +1,11 @@
+<?php
+declare(strict_types=1);
+
+namespace Shlinkio\Shlink\Core\Domain\Resolver;
+
+use Shlinkio\Shlink\Core\Entity\Domain;
+
+interface DomainResolverInterface
+{
+    public function resolveDomain(?string $domain): ?Domain;
+}

module/Core/src/Domain/Resolver/PersistenceDomainResolver.php

@@ -0,0 +1,27 @@
+<?php
+declare(strict_types=1);
+
+namespace Shlinkio\Shlink\Core\Domain\Resolver;
+
+use Doctrine\ORM\EntityManagerInterface;
+use Shlinkio\Shlink\Core\Entity\Domain;
+
+class PersistenceDomainResolver implements DomainResolverInterface
+{
+    /** @var EntityManagerInterface */
+    private $em;
+
+    public function __construct(EntityManagerInterface $em)
+    {
+        $this->em = $em;
+    }
+
+    public function resolveDomain(?string $domain): ?Domain
+    {
+        if ($domain === null) {
+            return null;
+        }
+
+        return $this->em->getRepository(Domain::class)->findOneBy(['authority' => $domain]) ?? new Domain($domain);
+    }
+}

module/Core/src/Domain/Resolver/SimpleDomainResolver.php

@@ -0,0 +1,14 @@
+<?php
+declare(strict_types=1);
+
+namespace Shlinkio\Shlink\Core\Domain\Resolver;
+
+use Shlinkio\Shlink\Core\Entity\Domain;
+
+class SimpleDomainResolver implements DomainResolverInterface
+{
+    public function resolveDomain(?string $domain): ?Domain
+    {
+        return $domain !== null ? new Domain($domain) : null;
+    }
+}