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/project.rs

@@ -0,0 +1,37 @@
+use serde::{Deserialize, Serialize};
+use serde_json::{Map, Value};
+
+#[derive(Debug, Clone, Default, Serialize, Deserialize)]
+pub struct Project {
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub id: Option<String>,
+    #[serde(default)]
+    pub client_id: String,
+    #[serde(default)]
+    pub name: String,
+    #[serde(default)]
+    pub number: String,
+    #[serde(default)]
+    pub task_rate: f64,
+    #[serde(default)]
+    pub due_date: String,
+    #[serde(default)]
+    pub budgeted_hours: f64,
+    #[serde(default)]
+    pub color: String,
+    #[serde(default)]
+    pub public_notes: String,
+    #[serde(default)]
+    pub private_notes: String,
+    #[serde(default)]
+    pub is_deleted: bool,
+    #[serde(default)]
+    pub archived_at: Option<i64>,
+    #[serde(default)]
+    pub created_at: Option<i64>,
+    #[serde(default)]
+    pub updated_at: Option<i64>,
+
+    #[serde(flatten)]
+    pub extra: Map<String, Value>,
+}