improve: OAuth2 UI and test suite (via #5066)

* create `features` folder

* add base oauth2 server

* continue implementing OAuth tests

* WIP

* add password flow tests

* modify Password flow credential types

* remove query string credential type

* add test case for Authorization flow

* add specific Authorization value for Password flow test

* WIP

* fix linter issues

test/e2e-cypress/plugins/index.js

@@ -1,3 +1,4 @@
+const startOAuthServer = require("../helpers/oauth2-server")
 // ***********************************************************
 // This example plugins/index.js can be used to load plugins
 //
@@ -12,6 +13,7 @@
 // the project's config changing)
 
 module.exports = (on, config) => {
+  startOAuthServer()
   // `on` is used to hook into various events Cypress emits
   // `config` is the resolved Cypress config
 }