Credits and billing

# Compute TOKEN as in /guides/authentication, then:
curl -s "$ENDPOINT/billing/credits" \
  -H "username: $EMAIL" \
  -H "token: $TOKEN"

<?php
// $endpoint, $email, $token computed as in /guides/authentication
$curl = curl_init("{$endpoint}/billing/credits");
curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
curl_setopt($curl, CURLOPT_HTTPHEADER, [
    "username: {$email}",
    "token: {$token}",
]);
echo curl_exec($curl);

# token(), EMAIL, ENDPOINT as in /guides/authentication
resp = requests.get(f"{ENDPOINT}/billing/credits",
                    headers={"username": EMAIL, "token": token()})
print(resp.json())

// token(), EMAIL, ENDPOINT as in /guides/authentication
const res = await fetch(`${ENDPOINT}/billing/credits`, {
  headers: { username: EMAIL, token: token() },
});
console.log(await res.json());

// token(), email, endpoint as in /guides/authentication
req, _ := http.NewRequest("GET", endpoint+"/billing/credits", nil)
req.Header.Set("username", email)
req.Header.Set("token", token())
resp, _ := http.DefaultClient.Do(req)
defer resp.Body.Close()
body, _ := io.ReadAll(resp.Body)
fmt.Println(string(body))