fix: fix issue with addWebRoute() api extension method (#2990)

* Fix regression that breaks the api.addWebRoute method

* Update Composer/packages/extensions/plugin-loader/src/composerPluginRegistration.ts

Co-authored-by: Andy Brown <asbrown002@gmail.com>

Co-authored-by: Andy Brown <asbrown002@gmail.com>
Co-authored-by: Chris Whitten <christopher.whitten@microsoft.com>

Composer/packages/extensions/plugin-loader/src/composerPluginRegistration.ts

@@ -121,7 +121,7 @@ export class ComposerPluginRegistration {
       const method = this.loader.webserver[type.toLowerCase()];
 
       if (typeof method === 'function') {
-        method(url, ...handlers);
+        method.call(this.loader.webserver, url, ...handlers);
       } else {
         throw new Error(`Unhandled web route type ${type}`);
       }