Consider adding axum::json!

[carllerche]

Feature Request

Consider adding an axum::json! macro that matches the serde_json macro syntax and avoids the stutter:

async fn hello_world() -> impl IntoResponse {
    axum::json!({ "hello": "world" })
}

Motivation

Returning JSON is common. This would avoid adding an extra dependency in the end user's Cargo.toml and avoid some stutter (Json(json!(...))) without exposing the serde_json dependency.