Implement spacetime login

[cloutiertyler]

Steps:

1. The CLI starts a websocket connection to https://spacetimedb.com/login-cli
2. It passes in a random nonce that it created to the websocket
3. Zeke figures out how to open the default browser and set the url to be https://spacetimedb.com/login-cli?nonce={the nonce}
4. The user logs in on the browser
5. The API backend sends the session token to the CLI over the websocket connection
6. The CLI provides the session token on every subsequent request to the website API
7. The CLI hits the /v1/identity endpoint to receive a SpacetimeDB token
8. The CLI provides this token to all SpacetimeDB requests (unless they are flagged --anonymous.

[bfops]

ChatGPT:

fn open_browser(url: &str) {
    let open_command = match env::consts::OS {
        "windows" => Command::new("cmd").args(&["/C", "start", url]),
        "macos" => Command::new("open").arg(url),
        "linux" => Command::new("xdg-open").arg(url),
        _ => {
            eprintln!("Unsupported OS");
            return;
        }
    };

    if let Err(e) = open_command.status() {
        eprintln!("Failed to open browser: {}", e);
    }
}

Instead of "unsupported OS", we'd probably just print the URL and wait to hear back on the websocket. Other CLIs do this.

[PuruVJ]

I saw that we also pull in this crate somewhere in spacetimedb https://crates.io/crates/webbrowser, so caninstead just reuse it