Add skeleton code for webhooks

Summary: Ref T11330. Adds general support for webhooks. This is still rough and missing a lot of pieces -- and not yet useful for anything -- but can make HTTP requests.

Test Plan: Used `bin/webhook call ...` to complete requests to a test endpoint.

Maniphest Tasks: T11330

Differential Revision: https://secure.phabricator.com/D19045

scripts/setup/manage_webhook.php

@@ -0,0 +1,21 @@
+#!/usr/bin/env php
+<?php
+
+$root = dirname(dirname(dirname(__FILE__)));
+require_once $root.'/scripts/init/init-script.php';
+
+$args = new PhutilArgumentParser($argv);
+$args->setTagline(pht('manage webhooks'));
+$args->setSynopsis(<<<EOSYNOPSIS
+**webhook** __command__ [__options__]
+    Manage webhooks.
+
+EOSYNOPSIS
+  );
+$args->parseStandardArguments();
+
+$workflows = id(new PhutilClassMapQuery())
+  ->setAncestorClass('HeraldWebhookManagementWorkflow')
+  ->execute();
+$workflows[] = new PhutilHelpArgumentWorkflow();
+$args->parseWorkflows($workflows);