Add support for environments with proxy servers

Our AWS environments require all requests to go through a proxy server; without support, our requests would get stuck.

We tackled this same problem in [Replace existing proxy config with global-agent](DEFRA/sroc-charging-module-api#572) in the sroc-charging-module-api. In the Charging Module, we used [global-agent](https://github.com/gajus/global-agent). This intercepts all http/https requests and adds the proxy details to them. Perfect!

The problem is it was last updated more than a year ago, the build is marked as failing, test coverage is low, and issues and PR's seem to be going unanswered. Our other issue is it does not recognise the lowercase versions of standard proxy env vars (`http_proxy`, `https_proxy` and `no_proxy`). It's a shame because it works seamlessly in the background.

The folks behind the request package we do use, [Got](https://github.com/sindresorhus/got), recommend a couple. We liked [hpagent](https://github.com/delvedor/hpagent/). The example was simple, it's being regularly updated, supports [keep-alive connections](https://en.wikipedia.org/wiki/HTTP_persistent_connection), and even uses [standardjs](https://standardjs.com/)!

So, this change supports working behind a proxy using **hpagent**.

app/lib/request.lib.js

@@ -4,6 +4,8 @@
  * @module RequestLib
  */
 
+const { HttpsProxyAgent } = require('hpagent')
+
 const requestConfig = require('../../config/request.config.js')
 
 async function get (url, additionalOptions = {}) {
@@ -48,6 +50,14 @@ async function _importGot () {
  */
 function _requestOptions (additionalOptions) {
   const defaultOptions = {
+    // This uses the spread operator and a logical AND short circuit evaluation to allow us to determine whether the
+    // following columns are added to the options or not. Thanks to https://stackoverflow.com/a/40560953/6117745 for
+    // this
+    ...(requestConfig.httpProxy && {
+      agent: {
+        https: new HttpsProxyAgent({ proxy: requestConfig.httpProxy })
+      }
+    }),
     // If we don't have this setting Got will throw its own HTTPError unless the result is 2xx or 3xx. That makes it
     // impossible to see what the status code was because it doesn't get set on the response object Got provides when
     // an error is thrown. With this set Got will treat a 404 in the same way it treats a 204.

config/request.config.js

@@ -1,7 +1,8 @@
 'use strict'
 
 const config = {
-  timeout: parseInt(process.env.REQUEST_TIMEOUT) || 5000
+  timeout: parseInt(process.env.REQUEST_TIMEOUT) || 5000,
+  httpProxy: process.env.http_proxy || 'http:localhost:1100'
 }
 
 module.exports = config

package.json

@@ -32,6 +32,7 @@
     "got": "^12.5.3",
     "govuk-frontend": "^4.4.0",
     "hapi-pino": "^11.0.1",
+    "hpagent": "^1.2.0",
     "knex": "^2.3.0",
     "nunjucks": "^3.2.3",
     "objection": "^3.0.1",