Adds a simple healthcheck endpoint for the items service

This change follows #736, and adds an HTTP healthcheck to the items API to ensure the scala service has started before it is registered healthy at the NLB and starts serving requests.

items/src/main/scala/weco/api/items/ItemsApi.scala

@@ -36,6 +36,14 @@ class ItemsApi(
         case id =>
           notFound(s"Work not found for identifier $id")
       }
+    },
+    pathPrefix("management") {
+      concat(
+        path("healthcheck") {
+          get {
+            complete("message" -> "ok")
+          }
+        })
     }
   )
 }