Send SMS with Rust

Add SMS sending to this Rust project with TextKit.

- Docs for this stack: https://jointextkit.com/docs/send-with-rust.md
- API: POST https://api.jointextkit.com/v1/messages with `Authorization: Bearer
  <key>` and a JSON body `{"to": "+1...", "body": "..."}`.
- There is no Rust crate: POST JSON to https://api.jointextkit.com/v1/messages
  with reqwest.
- Read the key from the `TEXTKIT_API_KEY` environment variable. Never hardcode
  it or send it to the browser.
- Phone numbers are E.164 and US or Canada only (`+1` followed by 10 digits).
- A test key (`ssms_sk_test_`) simulates: `+15005550006` always ends `delivered`
  and no phone gets a text.
- Errors are JSON: `{"error": {"code", "message", "param"}}` with a 4xx status.
  On `429`, wait `Retry-After` seconds and retry.
- Send only from server code that runs after a real event (a signup, an order).
  Never expose an endpoint that texts any number a visitor types.

export TEXTKIT_API_KEY=ssms_sk_test_...

cargo new send-sms && cd send-sms
cargo add reqwest --features json
cargo add tokio --features full
cargo add serde_json

use serde_json::{json, Value};

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let key = std::env::var("TEXTKIT_API_KEY")?;

    let res = reqwest::Client::new()
        .post("https://api.jointextkit.com/v1/messages")
        .bearer_auth(key)
        .json(&json!({ "to": "+15005550006", "body": "Hello from TextKit" }))
        .send()
        .await?;

    let status = res.status();
    let body: Value = res.json().await?;
    let text = |v: &Value| v.as_str().unwrap_or_default().to_string();
    if !status.is_success() {
        eprintln!("{}: {}", text(&body["error"]["code"]), text(&body["error"]["message"]));
        std::process::exit(1);
    }

    println!("{} {}", text(&body["id"]), text(&body["status"]));
    Ok(())
}

cargo run