[auth] add useSignInWithCustomToken hook

[mkhuda]

Some people manage projects with specifications requiring the use of custom secure JWT as the authentication method with Firebase, based on this documentation. Here I intend to add new useSignInWithCustomToken hook to your project.

But before, I thank this library a lot, which helped me in several React Firebase projects.

[yamcodes]

Great job! You'll want to export the hook from auth/index.ts so it is usable from the following import:

import { useSignInWithCustomToken } from "react-firebase-hooks/auth";

Note: I maintain a fork of this library called hooked-on-firebase that incorporates this PR and supports React 18.