Assistants: added RunStepList(), RunStep() and MessageRetrieve()

[pappde]

• RunService: added RunStepList() and RunStep()
• MessageService: added MessageRetrieve()

[pappde]

Notes

1. I tested in my own project, but did not add tests to the Playground project here. Can add if needed
2. polling "RunStepList()" needs logic to trigger when a new "step" is identified. The streaming implementation of runs might benefit from handling this in the SDK. However, I have not looked closely at the streaming events, so it's possible that this is moot in that scenario.

[kayhantolga]

Thank you very much for the PR, but I was also implementing this feature locally.