feat: allow custom broker client healthcheck path

The broker client healthcheck path can be customised by setting the
BROKER_HEALTHCHECK_PATH environment variable (or providing it in the
`.env` file), to be more accommodating of existing load balancers.

lib/client/index.js

@@ -16,7 +16,7 @@ module.exports = ({ port = null, config = {}, filters = {} }) => {
   const { app, server } = require('../webserver')(config, port);
 
   // IMPORTANT: defined before relay (`app.all('/*', ...`)
-  app.get('/healthcheck', (req, res, next) => {
+  app.get(config.brokerHealthcheckPath || '/healthcheck', (req, res) => {
     return res.status(200).json({ ok: true });
   });