feat: add backend status check handler middleware

The statusCheckHandler is a simple express middleware that helps
implement healthcheck/readiness endpoints. It can provided an optional
status function that should return a promise that resolves in either
true or false. The response will be either a 200 or 503 respectively.
Any errors that occur will be forwarded to next.

packages/backend/src/index.ts

@@ -25,6 +25,7 @@
 import {
   createServiceBuilder,
   getRootLogger,
+  statusCheckHandler,
   useHotMemoize,
 } from '@backstage/backend-common';
 import { ConfigReader, AppConfig } from '@backstage/config';
@@ -73,6 +74,7 @@ async function main() {
 
   const service = createServiceBuilder(module)
     .loadConfig(configReader)
+    .addRouter('/healthcheck', await statusCheckHandler())
     .addRouter('/catalog', await catalog(catalogEnv))
     .addRouter('/rollbar', await rollbar(rollbarEnv))
     .addRouter('/scaffolder', await scaffolder(scaffolderEnv))