feat(rest): add listenOnStart flag to control http listening for a rest server

The REST server can be mounted as a route for a facade Express application.
In such cases, we need to tell the REST server not to listen during start().

Author: raymondfeng

packages/rest/src/__tests__/integration/rest.server.integration.ts

@@ -143,6 +143,17 @@ describe('RestServer (integration)', () => {
     await server.stop();
   });
 
+  it('disables listening if noListen is set to true', async () => {
+    const server = await givenAServer({rest: {listenOnStart: false, port: 0}});
+    await server.start();
+    // No port is assigned
+    expect(server.getSync(RestBindings.PORT)).to.eql(0);
+    // No server url is set
+    expect(server.url).to.be.undefined();
+    expect(server.listening).to.be.false();
+    await server.stop();
+  });
+
   it('does not throw an error when stopping an app that has not been started', async () => {
     const server = await givenAServer();
     await expect(server.stop()).to.fulfilled();

packages/rest/src/rest.server.ts

@@ -853,6 +853,13 @@ export class RestServer extends Context implements Server, HttpServerLike {
     const protocol = await this.get(RestBindings.PROTOCOL);
     const httpsOptions = await this.get(RestBindings.HTTPS_OPTIONS);
 
+    if (this.config.listenOnStart === false) {
+      debug(
+        'RestServer is not listening as listenOnStart flag is set to false.',
+      );
+      return;
+    }
+
     const serverOptions = {};
     if (protocol === 'https') Object.assign(serverOptions, httpsOptions);
     Object.assign(serverOptions, {port, host, protocol, path});
@@ -1015,6 +1022,13 @@ export interface RestServerResolvedOptions {
   // eslint-disable-next-line @typescript-eslint/no-explicit-any
   expressSettings: {[name: string]: any};
   router: RestRouterOptions;
+
+  /**
+   * Set this flag to `false` to not listen on connections when the REST server
+   * is started. It's useful to mount a LoopBack REST server as a route to the
+   * facade Express application. If not set, the value is default to `true`.
+   */
+  listenOnStart?: boolean;
 }
 
 /**
@@ -1039,6 +1053,7 @@ const DEFAULT_CONFIG: RestServerResolvedConfig = {
   },
   expressSettings: {},
   router: {},
+  listenOnStart: true,
 };
 
 function resolveRestServerConfig(