Typed requests without needing the http crate

[itowlson]

This explores a possible approach to fermyon/spin-rust-sdk#23.

With this PR, a user can write a http_component entry point using an extractor, without needing the http crate (as this is not referenced in the template), by use-ing spin_sdk::http::typed::Request<T> instead of spin_sdk::http::Request. E.g.

// Note typed::Request rather than Request
use spin_sdk::http::{IntoResponse, typed::Request, Response, Json};

#[derive(Deserialize)]
struct Fie {
    thing: String,
    whatsit: i32,
}

#[http_component]
fn handle_test(req: Request<Json<Fie>>) -> anyhow::Result<impl IntoResponse> {
    let fie = req.body()?;
    let text = format!("The thing was {} and the whatsit was {}", fie.thing, fie.whatsit);
    Ok(Response::builder()
        .status(200)
        .header("content-type", "text/plain")
        .body(text)
        .build())
}

I don't love the duplication of methods - I don't know if this can be made more maintainable. But anyway putting this up for feedback and discussion.

[rylev]

I'm personally not convinced that it's worth the trouble. Turning a request body into a type is not too difficult (the user just needs to know how to turn a Vec<u8> into their type of choice), and the http crate is ubiquitous and well supported. Is it worth expanding the API of the SDK to support this extractor style directly? I'm not sure...

[fibonacci1729]

I'm similarly not entirely convinced this approach is worth it either ... I think I would be more in favor of an "extractor" as mentioned in the linked issue in the near term (which although is less aesthetic, provides reasonable convenience around having to go from serialized bytes to some user provided data structure).

Having said that, @itowlson I am amendable to what you're proposing here if you feel strongly about the DX.

[itowlson]

The people have spoken! Thanks for the feedback - I'll close.