Add subcription feature

shared/clients/payment_client.py

@@ -36,6 +36,8 @@ class Subscription:
     current_period_start: datetime
     current_period_end: datetime
     created_at: datetime
+    billing_cycle_anchor: Optional[datetime] = None
+    cancel_at_period_end: Optional[bool] = None
 
 
 @dataclass
@@ -81,9 +83,17 @@ class PaymentProvider(abc.ABC):
         pass
 
     @abc.abstractmethod
-    async def cancel_subscription(self, subscription_id: str) -> Subscription:
+    async def cancel_subscription(
+        self,
+        subscription_id: str,
+        cancel_at_period_end: bool = True
+    ) -> Subscription:
         """
         Cancel a subscription
+
+        Args:
+            subscription_id: Subscription ID to cancel
+            cancel_at_period_end: If True, cancel at end of billing period. Default True.
         """
         pass