Enhance type hinting in the subscr module

[troyswanson]

The subscr module defines classes related to CQN functionality. It would be really nice to see these classes fully decked out with type hints. Here are some ideas:

• Message.queries could return List[MessageQuery]
• Message.tables could return List[MessageTable]
• MessageTable.rows could return List[MessageRow]

These enhancements would make working with subscription message objects much easier. Since there are several rows of nesting involved with interacting with the actual rows, it's sometimes hard to keep track of where we are 💫

Thanks for your consideration!