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>

Cargo.toml

@@ -0,0 +1,16 @@
+[package]
+name = "invoiceninjars"
+version = "0.1.0"
+edition = "2021"
+description = "An async Rust client for the Invoice Ninja v5 API"
+license = "MIT"
+repository = "https://github.com/bryant/invoiceninjars"
+
+[dependencies]
+reqwest = { version = "0.12", default-features = false, features = ["json", "native-tls"] }
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+thiserror = "2"
+
+[dev-dependencies]
+tokio = { version = "1", features = ["rt-multi-thread", "macros"] }