feat(services): add monday-friday working day determiner

app/Services/WorkingDays/WorkingDayDeterminerInterface.php

@@ -0,0 +1,26 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Services\WorkingDays;
+
+/**
+ * Determinate if a given date is a working day.
+ */
+interface WorkingDayDeterminerInterface
+{
+    /**
+     * Determinate if a given date is a working day.
+     * @param \DateTimeImmutable $date
+     * @return bool
+     */
+    public function isWorkingDay(\DateTimeImmutable $date): bool;
+
+    /**
+     * Get working days count in a given interval.
+     * @param \DateTimeImmutable $startDate
+     * @param int $workingDays
+     * @return int
+     */
+    public function getWorkingDaysCount(\DateTimeImmutable $startDate, int $workingDays): int;
+}