Add `token-from-header` example for `axum`

[bramvanneerven]

Hi, since nearly all integrations have this example and I saw that someone was confused because of the API differences between the actix-web and axum integration (https://github.com/async-graphql/async-graphql/issues/977), I thought it would be nice to add this to the repository for axum as well 😄.

[sunli829]

Thanks! 🙂