feat: add method to retrieve the API usage so far in the cycle

[ngupta23]

Description

Add a method to the client to retrieve the API usage in the current cycle.

Use case

While experimenting, users may be interested in dynamically retrieving their API usage using the SDK to make tradeoffs with experiments and prod usage.

[ngupta23]

Per @Yibei990826, the following can be used in the mean time.

import requests

url = "https://api.nixtla.io/usage"

headers = {
  'Authorization': 'Bearer YOUR_API_KEY'
}

response = requests.request("GET", url, headers=headers)

print(response.text)

[jmoralez]

Fixed by #548