Adds a built-in PagerDuty action (elastic#43395)

The PagerDuty action can be used to post events via the PagerDuty
Events API v2:

https://v2.developer.pagerduty.com/docs/events-api-v2

x-pack/legacy/plugins/actions/server/builtin_action_types/index.ts

@@ -10,10 +10,12 @@ import { actionType as serverLogActionType } from './server_log';
 import { actionType as slackActionType } from './slack';
 import { actionType as emailActionType } from './email';
 import { actionType as indexActionType } from './es_index';
+import { actionType as pagerDutyActionType } from './pagerduty';
 
 export function registerBuiltInActionTypes(actionTypeRegistry: ActionTypeRegistry) {
   actionTypeRegistry.register(serverLogActionType);
   actionTypeRegistry.register(slackActionType);
   actionTypeRegistry.register(emailActionType);
   actionTypeRegistry.register(indexActionType);
+  actionTypeRegistry.register(pagerDutyActionType);
 }

x-pack/legacy/plugins/actions/server/builtin_action_types/lib/post_pagerduty.ts

@@ -0,0 +1,26 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License;
+ * you may not use this file except in compliance with the Elastic License.
+ */
+
+import axios, { AxiosResponse } from 'axios';
+import { Services } from '../../types';
+
+interface PostPagerdutyOptions {
+  apiUrl: string;
+  data: any;
+  headers: Record<string, string>;
+  services: Services;
+}
+
+// post an event to pagerduty
+export async function postPagerduty(options: PostPagerdutyOptions): Promise<AxiosResponse> {
+  const { apiUrl, data, headers } = options;
+  const axiosOptions = {
+    headers,
+    validateStatus: () => true,
+  };
+
+  return axios.post(apiUrl, data, axiosOptions);
+}