feat: do not ask for client secret when using auth code with PKCE (#7438)

Co-authored-by: Ignacio Lozano <nacholozano@gmail.com>
Co-authored-by: Vladimir Gorej <vladimir.gorej@gmail.com>

Refs #6290

test/e2e-cypress/tests/features/auth-code-flow-pkce-without-secret.js

@@ -0,0 +1,47 @@
+describe("Check client_secret for OAuth2 Authorization Code flow with and without PKCE (#6290)", () => {
+  it("should not display client_secret field for authorization code flow with PKCE", () => {
+    cy.visit(
+      "/?url=/documents/features/auth-code-flow-pkce-without-secret.yaml"
+    )
+      .window()
+      .then(win => {
+        // set auth config to use PKCE
+        let authConfigs = win.ui.authSelectors.getConfigs()
+        win.ui.authActions.configureAuth({
+          ...authConfigs,
+          usePkceWithAuthorizationCodeGrant: true,
+        })
+      })
+      .get("button.authorize")
+      .click()
+      .get("h4")
+      .contains("authorizationCode with PKCE")
+      .get(".flow")
+      .contains("authorizationCode with PKCE")
+      .get("#client_secret")
+      .should("not.exist")
+  })
+
+  it("should display client_secret field for authorization code flow without PKCE", () => {
+    cy.visit(
+      "/?url=/documents/features/auth-code-flow-pkce-without-secret.yaml"
+    )
+      .window()
+      .then(win => {
+        // set auth config to not use PKCE
+        let authConfigs = win.ui.authSelectors.getConfigs()
+        win.ui.authActions.configureAuth({
+          ...authConfigs,
+          usePkceWithAuthorizationCodeGrant: false,
+        })
+      })
+      .get("button.authorize")
+      .click()
+      .get("h4")
+      .contains("authorizationCode")
+      .get(".flow")
+      .contains("authorizationCode")
+      .get("#client_secret")
+      .should("exist")
+  })
+})