Implement REST API client adapter (vantage-api-client)

[romaninsh]

Adds vantage-api-client crate that implements TableSource for REST APIs using serde_json::Value directly — no custom type system needed, entities just use #[derive(Serialize, Deserialize)].

What's included

• RestApi struct with base URL and Bearer auth support
• TableSource impl that fetches paginated JSON ({"data": [...], "pagination": {...}})
• Standalone test server (test-api-server) serving cities.csv via warp
• Example with Country/City models demonstrating entity loading and reference traversal

Changes to existing crates

• ReadableDataSet::get() now accepts impl Into<Self::Id> — so you can write table.get("Germany") instead of table.get(&"Germany".to_string())
• Table::with_id_column() sets the ID field per-table (moved from data source level)
• print_table() now takes &Table directly, fetches data async, and reads the id field from column flags to skip duplicate columns

Example usage

let countries = Country::api_table();
print_table(&countries).await?;

let argentina: Country = countries.get("Argentina").await?;
let cities = argentina.ref_cities(); // fetches /countries/Argentina/cities
print_table(&cities).await?;

let rosario: City = cities.get("Rosario").await?;
println!("population: {}", rosario.population);