Feat/xemm gateway

This PR adds an option to use DEX as taker exchanges through the gateway in the XEMM strategy.

Link to PR: https://github.com/hummingbot/hummingbot/pull/5550

Tests performed by the developer:

Added new unit tests Ran all existing unit tests Tested with binance paper & uniswap (kovan) Tested with binance paper & pangolin (fuji) Tested with binance & kucoin

Tips for QA testing:

Please test thoroughly both with CEX and DEX.