Allow setting the callbackUrl for signin in a custom page form without using custom handler

[humont]

Description 📓

A nice-to-have feature would be the ability to set the callbackUrl as a field in the form on a custom signIn page.

using the example from the documentation:

import { getCsrfToken } from "next-auth/react"

export default function SignIn({ csrfToken }) {
  return (
    <form method="post" action="/api/auth/signin/email">
      {/**Being able to add the following hidden input, and have it parsed on the server and added to the redirect on succesful signins: */}
      <input name="callbackUrl" type="hidden" defaultValue='/custom/callback/url' />
      <input name="csrfToken" type="hidden" defaultValue={csrfToken} />
      <label>
        Email address
        <input type="email" id="email" name="email" />
      </label>
      <button type="submit">Sign in with Email</button>
    </form>
  )
}

// This is the recommended way for Next.js 9.3 or newer
export async function getServerSideProps(context) {
  const csrfToken = await getCsrfToken(context)
  return {
    props: { csrfToken },
  }
}

/*
// If older than Next.js 9.3
SignIn.getInitialProps = async (context) => {
  return {
    csrfToken: await getCsrfToken(context)
  }
}
*/

How to reproduce ☕️

Currently the only way to specify a custom callbackUrl is by using the signin('email', {callbackUrl: '/someurl'}) hook.

Contributing 🙌🏽

Yes, I am willing to help implement this feature in a PR