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>

src/models/mod.rs

@@ -0,0 +1,19 @@
+mod client;
+mod common;
+mod expense;
+mod invoice;
+mod payment;
+mod product;
+mod project;
+mod task;
+mod vendor;
+
+pub use client::{Client, ClientContact};
+pub use common::{ItemResponse, ListParams, ListResponse, Meta, Pagination};
+pub use expense::Expense;
+pub use invoice::{Credit, Invoice, InvoiceItem, Quote};
+pub use payment::{Payment, PaymentInvoice};
+pub use product::Product;
+pub use project::Project;
+pub use task::Task;
+pub use vendor::{Vendor, VendorContact};