#5638 Feat/update okx connector

This is a general update of the OKX connector to make it compliant with all latest standards for connectors. The task also includes the following changes intended to improve the process to create new centralized connectors: Refactoring of order book and user stream data source classes to move common code to the super class. The refactoring was applied to Binance, Kucoin, GateIO and OKX connectors. The logic that was previously in the OrderBookDataSource class to get all trading pairs and the last traded prices is moved with this PR to the connector class. The change was also applied to Binance, Kucoin, GateIO and OKX connectors. All other existing connectors have been updated so that the connector class implements the required methods, but the logic that gets the trading pairs and the traded prices is still in their OrderBookDataSource classes. Created a new abstract test class for exchange classes that includes all the basic tests. The intention is that new connectors will be able to create the exchange class tests by subclassing the abstract test class and implementing all abstract methods intended to return the mock responses from the server with the appropriate format.

PR: https://github.com/hummingbot/hummingbot/pull/5368