Get rate for a trading pair using async fetch (direct from exchange).
This bypasses the cached prices and fetches directly from the source. Useful when cached data may be stale or not yet initialized.
Args: trading_pair: Trading pair in format BASE-QUOTE (e.g., BTC-USDT)
Returns: Rate for the specified trading pair
Basic authentication header of the form Basic <encoded-value>, where <encoded-value> is the base64-encoded string username:password.