Turbopack: Make turbo-tasks-fetch a bit more OOP-like (#81995)
This was a request from @lukesandberg to make this API a bit more object-oriented: https://github.com/vercel/next.js/pull/81818#discussion_r2223136594
- `ReqwestClientConfig` is now just `FetchClient`.
- `fetch` is a method on `FetchClient` instead of a function taking a config.
- **Bonus change:** `lib.rs` is now a tiny stub that just re-exports things from other modules (`client.rs`, `error.rs`, `response.rs`).
