Initial commit: async Rust client for the Invoice Ninja v5 API

Covers Clients, Invoices, Quotes, Credits, Payments, Products, Expenses,
Projects, Tasks, and Vendors, each with list, list_all, get, create,
update, and delete, generated by the resource_methods! macro.

Every model carries a #[serde(flatten)] extra map so fields the API
returns but the crate does not model survive a get/update round-trip
instead of being silently dropped.

Includes a Nix dev shell (Rust via rust-overlay), a GitHub Actions CI
workflow running build/test/fmt/clippy, and round-trip tests for the
extra-preservation invariant.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

examples/list_clients.rs

@@ -0,0 +1,21 @@
+use invoiceninjars::models::ListParams;
+use invoiceninjars::InvoiceNinjaClient;
+
+#[tokio::main]
+async fn main() -> invoiceninjars::Result<()> {
+    let base_url =
+        std::env::var("IN_BASE_URL").unwrap_or_else(|_| "https://invoicing.co/api/v1".into());
+    let token = std::env::var("IN_API_TOKEN").expect("set IN_API_TOKEN");
+
+    let client = InvoiceNinjaClient::new(base_url, token)?;
+
+    let clients = client
+        .list_clients(&ListParams::new().per_page(10).sort("created_at|desc"))
+        .await?;
+
+    for c in clients {
+        println!("{:?}\t{}", c.id, c.name);
+    }
+
+    Ok(())
+}