Add composable middleware

[PaulAsjes]

Fixes #47

Right now we only give users the option to use authkitMiddleware as a drop-in middleware replacement. This works fine if you only want to use middleware for auth purposes, but in more complex apps you might want to stitch together multiple middleware functionalities.

This introduces authkit as a middleware function that allows you to compose middleware yourself at the expense of some of the automatic redirecting and "secure by default" functionality of authkitMiddleware.

Example use:

export default async function middleware(request: NextRequest) {
  // Perform logic before or after AuthKit
  
  // Auth object contains the session, response headers and an auhorization URL in the case that the session isn't valid
  // This method will automatically handle setting the cookie and refreshing the session
  const auth = await authkit(request, {
    debug: true,
  });

  // Control of what to do when there's no session on a protected route is left to the developer
  if (request.url.includes("/account") && !auth.session.user) {
    console.log("No session on protected path");
    return NextResponse.redirect(auth.authorizationUrl);
    
    // Alternatively you could redirect to your own login page, for example if you want to use your own UI instead of hosted AuthKit
    return NextResponse.redirect("/login");
  }

  // Headers need to be included in every non-redirect response to ensure that `withAuth` works as expected
  return NextResponse.next({
    headers: auth.headers,
  });
}

// Match against the pages
export const config = { matcher: ["/", "/account/:path*", "/api/:path*"] };

[PaulAsjes]

Next always stores its cookies in the format

{
  name: string;
  value: string;
  ...other options
}

so changed this to reflect what next actually does.