add workflow engine interface (#25)

What changed?

WorkflowEngine is a stateful entity that process decision tasks and returns new decisions. It's just a fancy name of Decider in java SDK.

Author: shijiesheng

cadence/workflow/workflow_engine.py

@@ -0,0 +1,28 @@
+from dataclasses import dataclass
+from typing import Callable
+
+from cadence.api.v1.decision_pb2 import Decision
+from cadence.client import Client
+from cadence.data_converter import DataConverter
+from cadence.api.v1.service_worker_pb2 import PollForDecisionTaskResponse
+
+@dataclass
+class WorkflowContext:
+    domain: str
+    workflow_id: str
+    run_id: str
+    client: Client
+    workflow_func: Callable
+    data_converter: DataConverter
+
+@dataclass
+class DecisionResult:
+    decisions: list[Decision]
+
+class WorkflowEngine:
+    def __init__(self, context: WorkflowContext):
+        self._context = context
+
+    # TODO: Implement this
+    def process_decision(self, decision_task: PollForDecisionTaskResponse) -> DecisionResult:
+        return DecisionResult(decisions=[])