feat: add support for mounting auth routes as api routes

[frederikprijck]

📋 Changes

This PR adds support for mounting the authentication routes with both Next.js' App Router and Pages Router, enabling developers to use our SDK without handling these in the middleware.

Usage

This PR introduces a new handleAuth method that works seamlessly with both routing patterns:

App Router usage:

// app/api/auth/[...auth0]/route.ts
import { auth0 } from "@/lib/auth0";

export const GET = auth0.handleAuth;
export const POST = auth0.handleAuth;

Pages Router usage:

// pages/api/auth/[...auth0].ts
import { auth0 } from "@/lib/auth0";

export default auth0.handleAuth;

Technical Changes

• Enhanced AuthClient.handler (internal) method with method overloading to support both NextRequest (App Router/Middleware) and NextApiRequest (Pages Router)
• Added new HTTP adapters (Auth0NextApiRequest, Auth0NextApiResponse, Auth0ApiResponseCookies) to handle Pages Router request/response objects
• Extended OnCallbackHook context to include request and response objects for Pages Router compatibility
• Added runtime detection using isRequest() to automatically determine the routing pattern and use appropriate handlers

📎 References

N/A

🎯 Testing

[frederikprijck]

In App Router, users can return a NextResponse instance from onCallback. However, in Pages Router, the user should mutate the original NextApiResponse, so we need to expose response when using Pages Router.

Exposing request made things a bit simpler for the defaultOnCallback we have, and has been requested here, so feels like a useful thing to consider.

[frederikprijck]

As our SDK has a capability to expose certain proxy endpoints, we keep these as part of the proxy integration.

When using our middleware, it would mount both the authentication routes and the proxy routes automatically. With this PR, authentication routes can be mounted through the API routes with either Pages or App Router, which does not include the proxy functionality. That needs to be configured in the customers proxy.ts file, by calling handleProxy().