ref(transport): Add flush_async in the Transport abc

Add a new flush_async method in the Transport ABC. This is needed for the async transport, as calling it from the client
while preserving execution order in close will require flush to be a coroutine, not a function.

GH-4568

Author: srothh

sentry_sdk/transport.py

@@ -107,6 +107,19 @@ def flush(
         """
         return None
 
+    async def flush_async(
+        self: Self,
+        timeout: float,
+        callback: Optional[Any] = None,
+    ) -> None:
+        """
+        Send out current events within `timeout` seconds. This method needs to be awaited for blocking behavior.
+
+        The default implementation is a no-op, since this method may only be relevant to some transports.
+        Subclasses should override this method if necessary.
+        """
+        return None
+
     def kill(self: Self) -> None:
         """
         Forcefully kills the transport.