gcpaccesstoken - rust

this is a pure rust implimentation to get google cloud access token for servers with service account credentials.

```rust use gcpaccesstoken;

[tokio::main]

async fn main() {

match gcp_access_token::generator::init("./credentials.json".to_string()).await{
    Ok(v)=>{
        println!("token successfull : {}",v);
    },
    Err(_e)=>{
        println!("!!! failed-token : {:?}",_e);
    }
}

}

```