Add CRUD example

[lz1998]

I have written Book-Management with 6 handlers: register, login, create_book, search_book, update_book and delete_book.

These handlers can help beginners learn how to write a simple CRUD website with axum, sea-orm, jwt.

Can I create a PR to add this example?

[davidpdrsn]

Depends on how big it is. Since examples also takes time to maintain I would like to keep them small. So it might be better to link to it from here https://github.com/tokio-rs/axum/blob/main/ECOSYSTEM.md