Add PHPstan and fix all checks for max level

src/Client/ClientInterface.php

@@ -5,9 +5,10 @@ namespace gullevek\AmazonIncentives\Client;
 interface ClientInterface
 {
 	/**
-	 * @param string       $url     The URL being requested, including domain and protocol
-	 * @param array        $headers Headers to be used in the request
-	 * @param array|string $params  Can be nested for arrays and hashes
+	 * @param string              $url     The URL being requested,
+	 *                                     including domain and protocol
+	 * @param array<mixed>        $headers Headers to be used in the request
+	 * @param array<mixed>|string $params  Can be nested for arrays and hashes
 	 *
 	 * @return String
 	 */