feat: Support scheduling Activities within Workflows

[natemort]

What changed?

• Add preliminary structure for scheduling activities within Workflow.
• Invoking activity functions within a Workflow will schedule them as an activity.
• DataConverter is no longer async so that it can be used outside of the main event loop.
• CadenceError -> CadenceRpcError to indicate that it's the base error for all Rpc errors, not all errors the client may raise.
• State machine restructure
  • State machines are split into multiple files and moved to workflow.statemachine
  • HistoryEvents are dispatched to state machines based on their class and a field specified in the decorator. This resolves the ambiguity in handling events and the custom handling needed for each type.
  • DecisionManager stores all state machines in a single ordered dictionary, and reorders it when state machines are manipulated. This is required for the decisions to be ordered correctly.
  • State machines interact with futures, completing them based on the history
  • Canceling the futures will cancel the activities/timers

Why?

• Core workflow functionality

How did you test it?

• Unit tests. We need the deterministic event loop and workflow execution supported before this can effectively be tested.

Potential risks

Release notes

Documentation Changes

[timl3136]

Why do we need this field?

[shijiesheng]

I remember the original reason for making this async is to support blob retrieval. Is it no longer needed?

[natemort]

If the DataConverter is async we can only use it within the main event loop. It's possible to keep it async but we would have to restructure how and when we convert values. When applying history events for example, we would need to convert all the resulting payloads first and then use them within the state machines.

I think we can explore making it async or introducing another concept in the future. For example, we could add some sort of processor that takes a Payload and returns a new Payload, potentially compressing, encoding, or even downloading the payload from a path. Then the converter still just operates on bytes.