Get rates for specified trading pairs.
Uses the configured rate oracle source to fetch current rates.
Args: rate_request: List of trading pairs to get rates for
Returns: Rates for the requested trading pairs
Basic authentication header of the form Basic <encoded-value>, where <encoded-value> is the base64-encoded string username:password.
Request for getting rates.
List of trading pairs to get rates for (e.g., ['BTC-USDT', 'ETH-USDT'])