Add async send_async method to support Async/Await for SMS services

[ipkiruiYegon]

[ipkiruiYegon]

import africastalking

africastalking.initialize(username, api_key)
sms = africastalking.SMS

async def send_message(message, mobile_no):
    response = await (sms.send_async(message, [mobile_no]))
    return response