C02 API Basic Rust Implementation

DONE: - Basic API implementation - Client Format - Two C02 request implementations.

TODO: - Rate Limitation - Support of future Electricity Maps commercial API

Example:

```rust use rustc02signal;

async fn dostuff() -> () { let c02client = rustc02signal::Client::new("API-KEY".tostring());

// Get the latest data for a country code
let resp = c02client.latest_cc("GB".to_string()).await;

println!("{:?}", resp);

// Get the latest data for a set of long/lat coords
let resp = c02client.latest_gc(51.5074, 0.1278).await;

println!("{:?}", resp);

} ```