feat: Implement IntoResponseParts for Redirect

[HaiqalAly]

Motivation

I saw the feature request in #3637 and wanted to try helping out!

The goal is to allow Redirect to be used as part of a response tuple, which makes it much easier to provide a custom body alongside a redirect. Currently, you have to manually build the response with StatusCode and headers to achieve this.

Solution

I've implemented IntoResponseParts for Redirect, so it can now be used like this:

(
    Redirect::to(EXAMPLE_URL),
    Html(format!(
        r#"<p>Redirecting to <a href="{EXAMPLE_URL}">{EXAMPLE_URL}</a></p>"#
    )),
)

To make this work, I had to add a set_status method to ResponseParts in axum-core, as I couldn't find a way to modify the status code otherwise.

umm... Just realized what I did and I admit I felt scared because I touched the core crate! 😅 I wasn't sure if axum-core changes are strictly reserved for maintainers or if there are stability guarantees I might be unknowingly breaking. I think this change is safe since it's purely additive, but please let me know if I've overstepped or if there's a different way you prefer this to be handled. I'm very happy to close this or adjust it based on your guidance.