Add support for axum's IntoResponse

[mscofield0]

Currently I have an endpoint that just makes requests to individual services and there's no way currently to bridge between the two Response types.

This could be fixed by adding a into_http() or as_http() to the Response type.

[1oglop1]

It would be great... I spent a week learning Rust and doing exactly this to send 1 request and return its response.
I am converting the incoming request body (bytes) into reqwest::Body::from(raw_body); and then building the response from reqwest::Response attributes.

[shouya]

I added the conversion from reqwest::Response to http::Response<reqwest::Body> as well as reqwest::Body as http_body::Body in #2060. That PR should fix this issue.