ref(worker): Add flush_async method to Worker ABC

Add a new flush_async method to worker ABC. This is necessary because the async transport cannot use a
synchronous blocking flush.

GH-4578

Author: srothh

sentry_sdk/worker.py

@@ -33,16 +33,29 @@ def is_alive(self) -> bool:
     def kill(self) -> None:
         pass
 
-    @abstractmethod
     def flush(
         self, timeout: float, callback: Optional[Callable[[int, float], None]] = None
     ) -> None:
         """
         Flush the worker.
 
         This method blocks until the worker has flushed all events or the specified timeout is reached.
+        Default implementation is a no-op, since this method may only be relevant to some workers.
+        Subclasses should override this method if necessary.
         """
-        pass
+        return None
+
+    async def flush_async(
+        self, timeout: float, callback: Optional[Callable[[int, float], None]] = None
+    ) -> None:
+        """
+        Flush the worker.
+
+        This method can be awaited until the worker has flushed all events or the specified timeout is reached.
+        Default implementation is a no-op, since this method may only be relevant to some workers.
+        Subclasses should override this method if necessary.
+        """
+        return None
 
     @abstractmethod
     def full(self) -> bool: