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/tests/features/oauth2-flows/application.js

@@ -0,0 +1,55 @@
+describe("OAuth2 Application flow", function() {
+  beforeEach(() => {
+    cy.server()
+    cy.route({
+      url: "**/oauth/*",
+      method: "POST"
+    }).as("tokenRequest")
+  })
+
+  it("should make an application flow Authorization header request", () => {
+    cy
+      .visit("/?url=http://localhost:3231/swagger.yaml")
+      .get(".btn.authorize")
+      .click()
+
+      .get("div.modal-ux-content > div:nth-child(2)").within(() => {
+        cy.get("#client_id")
+          .clear()
+          .type("confidentialApplication")
+
+          .get("#client_secret")
+          .clear()
+          .type("topSecret")
+
+          .get("button.btn.modal-btn.auth.authorize.button")
+          .click()
+      })
+
+    cy.get("button.close-modal")
+      .click()
+
+      .get("#operations-default-get_application")
+      .click()
+
+      .get(".btn.try-out__btn")
+      .click()
+
+      .get(".btn.execute")
+      .click()
+
+    cy.get("@tokenRequest")
+      .its("request")
+      .its("body")
+      .should("equal", "grant_type=client_credentials")
+
+    cy.get("@tokenRequest")
+      .its("request")
+      .its("headers")
+      .its("authorization")
+      .should("equal", "Basic Y29uZmlkZW50aWFsQXBwbGljYXRpb246dG9wU2VjcmV0")
+
+      .get(".live-responses-table .response-col_status")
+      .contains("200")
+  })
+})