Using the `Json` extractor with the 2.1 Rust template

[itowlson]

In Spin 2.0.1, the Rust template referenced the http crate, and we supported the Json<T> extractor as a body type parameter e,g, http::Request<Json<Person>>.

In Spin 2.1, the Rust template does not reference the http crate and users are directed instead to the spin_sdk::http::Request type. This type is not generic - the body is always presented as bytes - and we do not have seem to have samples of how to use the extractors with it. I guess you can do something like Jsion::<Person>::try_from_body(req.into_body()) but it's not as friendly as the extractor syntax!

If the extractors work with the SDK request type, it would be good to provide samples that show how; if they don't, we should provide that functionality.

[rylev]

The following used to work in the run up to 2.0 but was removed because of some fears that it was potentially confusing:

use std::collections::HashMap;
use spin_sdk::{http::{Json, Response}, http_component};

/// A simple Spin HTTP component.
#[http_component]
fn handle(body: Json<HashMap<String, String>>) -> Response {
    println!("Incoming Body: {body:#?}");
    Response::new(200, Vec::new())
}

We could make this work again if we want, but of course, the user then doesn't have access to any other parts of the request.

Alternatively, we could add a json method to Request that allows users to access the body in some deserialized form.

[itowlson]

A json method feels very specific (although admittedly the "80% case" kind of specific, with String and maybe form-encoded being the other biggies). I had also wondered about a function called something like into_body_as() or extract, but the req.extract::<Json<Person>>() syntax is a bit spiky.

Another, although maybe hairy, possibility is a typed form of request e.g. spin_sdk::http::typed::Request<T>. It could be a testing and maintenance pain though, and of course a discoverability issue and extra cognitive load for users.