feat(services): add database storage for public holidays

app/Services/WorkingDays/PublicHolidays/PublicHolidaysStateStorageInterface.php

@@ -0,0 +1,20 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Services\WorkingDays\PublicHolidays;
+
+use App\Models\NonWorkingDays;
+
+/**
+ * Storage for storing public holidays in persistent storage.
+ */
+interface PublicHolidaysStateStorageInterface
+{
+    /**
+     * Store a public holiday in the storage.
+     * @param \DateTimeImmutable $publicHolidayDate
+     * @return NonWorkingDays|null
+     */
+    public function storePublicHoliday(\DateTimeImmutable $publicHolidayDate): ?NonWorkingDays;
+}