Add samples for Invoice Template API

sample/invoice-templates/GetAllInvoiceTemplates.php

@@ -0,0 +1,18 @@
+<?php
+
+use PayPal\Api\Templates;
+
+require 'CreateInvoiceTemplate.php';
+
+try {
+    $templates = Templates::getAll(array("fields" => "all"), $apiContext);
+}  catch (Exception $ex) {
+    // NOTE: PLEASE DO NOT USE RESULTPRINTER CLASS IN YOUR ORIGINAL CODE. FOR SAMPLE ONLY
+    ResultPrinter::printError("Get all Templates", "Templates", null, null, $ex);
+    exit(1);
+}
+
+// NOTE: PLEASE DO NOT USE RESULTPRINTER CLASS IN YOUR ORIGINAL CODE. FOR SAMPLE ONLY
+ResultPrinter::printResult("Get all Templates", "Templates", null, null, $templates);
+
+return $templates;