better typing for streamer, orders

[Graeme22]

Description

Improves typing for orders and complex orders, where union types make things very unpleasant at times.
Improves typing for alert and data streamers, so that types are correct when using listen, get_event, etc.

Pre-merge checklist

• Code formatted correctly (check with make lint)
• Code implemented for both sync and async
• Passing tests locally (check with make test, make sure you have TT_USERNAME, TT_PASSWORD, and TT_ACCOUNT environment variables set)
• New tests added (if applicable)

Please note that, in order to pass the tests, you'll need to set up your Tastytrade credentials as repository secrets on your local fork. Read more at CONTRIBUTING.md.