feat: Support scheduling Activities within Workflows (cadence-workflow#41)

<!-- Describe what has changed in this PR -->
**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

<!-- Tell your future self why have you made these changes -->
**Why?**
- Core workflow functionality

<!-- How have you verified this change? Tested locally? Added a unit
test? Checked in staging env? -->
**How did you test it?**
- Unit tests. We need the deterministic event loop and workflow
execution supported before this can effectively be tested.

<!-- Assuming the worst case, what can be broken when deploying this
change to production? -->
**Potential risks**

<!-- Is it notable for release? e.g. schema updates, configuration or
data migration required? If so, please mention it, and also update
CHANGELOG.md -->
**Release notes**

<!-- Is there any documentation updates should be made for config,
https://cadenceworkflow.io/docs/operation-guide/setup/ ? If so, please
open an PR in https://github.com/cadence-workflow/cadence-docs -->
**Documentation Changes**

Author: natemort

cadence/_internal/activity/_activity_executor.py

@@ -73,7 +73,7 @@ async def _report_failure(
             _logger.exception("Exception reporting activity failure")
 
     async def _report_success(self, task: PollForActivityTaskResponse, result: Any):
-        as_payload = await self._data_converter.to_data([result])
+        as_payload = self._data_converter.to_data([result])
 
         try:
             await self._client.worker_stub.RespondActivityTaskCompleted(

cadence/_internal/activity/_context.py

@@ -19,13 +19,13 @@ def __init__(
         self._activity_fn = activity_fn
 
     async def execute(self, payload: Payload) -> Any:
-        params = await self._to_params(payload)
+        params = self._to_params(payload)
         with self._activate():
             return await self._activity_fn(*params)
 
-    async def _to_params(self, payload: Payload) -> list[Any]:
+    def _to_params(self, payload: Payload) -> list[Any]:
         type_hints = [param.type_hint for param in self._activity_fn.params]
-        return await self._client.data_converter.from_data(payload, type_hints)
+        return self._client.data_converter.from_data(payload, type_hints)
 
     def client(self) -> Client:
         return self._client
@@ -46,7 +46,7 @@ def __init__(
         self._executor = executor
 
     async def execute(self, payload: Payload) -> Any:
-        params = await self._to_params(payload)
+        params = self._to_params(payload)
         loop = asyncio.get_running_loop()
         return await loop.run_in_executor(self._executor, self._run, params)