Add a JSON extractor

[dzamlo]

It can be useful to accept JSON in request.

Here is a minimal implementation using serde_json_core

use serde_json_core::de::Error as JsonError;

pub struct JsonRejection(JsonError);

impl IntoResponse for JsonRejection {
    async fn write_to<W: picoserve::response::ResponseWriter>(
        self,
        response_writer: W,
    ) -> Result<picoserve::ResponseSent, W::Error> {
        (
            picoserve::response::status::BAD_REQUEST,
            format_args!("Request Body is not valid JSON: {}", self.0),
        )
            .write_to(response_writer)
            .await
    }
}

pub struct Json<T: serde::de::DeserializeOwned>(pub T);

impl<State, T: serde::de::DeserializeOwned> FromRequest<State> for Json<T> {
    type Rejection = JsonRejection;

    async fn from_request(
        _state: &State,
        request: &Request<'_>,
    ) -> Result<Json<T>, JsonRejection> {
        serde_json_core::from_slice(request.body)
            .map(|(v, _)| Self(v))
            .map_err(JsonRejection)
    }
}

[sammhicks]

Looks good! I'm waiting on rust-embedded-community/serde-json-core#79 at the moment, but once it's merged, I'm planning on supporting parsing JSON bodies

[vpikulik]

please have a look: https://github.com/sammhicks/picoserve/pull/33/files