Create new API endpoint for WC coupons

Because I haven't found an easy way how to use WP or WC API,
I decided to create a MailPoet endpoint for getting coupons that
allows me loading them via Ajax.
[MAILPOET-5123]

mailpoet/lib/API/JSON/v1/Coupons.php

@@ -0,0 +1,65 @@
+<?php declare(strict_types = 1);
+
+namespace MailPoet\API\JSON\v1;
+
+use MailPoet\API\JSON\Endpoint as APIEndpoint;
+use MailPoet\API\JSON\SuccessResponse;
+use MailPoet\Config\AccessControl;
+use MailPoet\WooCommerce\Helper;
+use MailPoet\WP\Functions as WPFunctions;
+
+class Coupons extends APIEndpoint {
+  public const DEFAULT_PAGE_SIZE = 10;
+
+  /** @var Helper  */
+  public $helper;
+
+  /*** @var WPFunctions */
+  private $wp;
+
+  public $permissions = [
+    'global' => AccessControl::PERMISSION_MANAGE_EMAILS,
+  ];
+
+  public function __construct(
+    WPFunctions $wp,
+    Helper $helper
+  ) {
+    $this->wp = $wp;
+    $this->helper = $helper;
+  }
+
+  public function getCoupons(array $data = []): SuccessResponse {
+    $pageSize = $data['page_size'] ?? self::DEFAULT_PAGE_SIZE;
+    $pageNumber = $data['page_number'] ?? 0;
+    $discountType = $data['discount_type'] ?? null;
+    $search = $data['search'] ?? null;
+    $includeCouponId = $data['include_coupon_id'] ?? null;
+
+    return $this->successResponse(
+      $this->formatCoupons($this->helper->getCouponList(
+        (int)$pageSize,
+        (int)$pageNumber,
+        $discountType,
+        $search,
+        (int)$includeCouponId
+      ))
+    );
+  }
+
+  /**
+   * @param array $couponPosts
+   * @return array
+   */
+  private function formatCoupons(array $couponPosts): array {
+    return array_map(function (\WP_Post $post): array {
+      $discountType = $this->wp->getPostMeta($post->ID, 'discount_type', true);
+      return [
+        'id' => $post->ID,
+        'text' => $post->post_title, // phpcs:ignore Squiz.NamingConventions.ValidVariableName.MemberNotCamelCaps
+        'excerpt' => $post->post_excerpt, // phpcs:ignore Squiz.NamingConventions.ValidVariableName.MemberNotCamelCaps
+        'discountType' => $discountType,
+      ];
+    }, $couponPosts);
+  }
+}