refactor: update Gateway contract to improve fee handling and add aggregator address retrieval

- Renamed variables for clarity in fee calculations, changing `senderAmount` to `amountToSettle`.
- Updated logic to correctly handle aggregator fees during order settlements.
- Added a new function `getAggregator` in the IGateway interface to retrieve the aggregator address.
- Adjusted event emissions to reflect changes in fee handling.

Author: chibie