feat: Implement RoutingPublisher which routes between publishers. (#10)

* feat: Implement SerialBatcher which helps with transforming single writes into batch writes.

* feat: Implement SinglePartitionPublisher which publishes to a single partition and handles retries.

* feat: Implement RoutingPublisher which routes between publishers.

google/cloud/pubsublite/internal/wire/routing_publisher.py

@@ -0,0 +1,29 @@
+from typing import Dict
+
+from google.cloud.pubsublite.internal.wire.publisher import Publisher
+from google.cloud.pubsublite.internal.wire.routing_policy import RoutingPolicy
+from google.cloud.pubsublite.partition import Partition
+from google.cloud.pubsublite.publish_metadata import PublishMetadata
+from google.cloud.pubsublite_v1 import PubSubMessage
+
+
+class RoutingPublisher(Publisher):
+  _routing_policy: RoutingPolicy
+  _publishers: Dict[Partition, Publisher]
+
+  def __init__(self, routing_policy: RoutingPolicy, publishers: Dict[Partition, Publisher]):
+    self._routing_policy = routing_policy
+    self._publishers = publishers
+
+  async def __aenter__(self):
+    for publisher in self._publishers.values():
+      await publisher.__aenter__()
+
+  async def __aexit__(self, exc_type, exc_val, exc_tb):
+    for publisher in self._publishers.values():
+      await publisher.__aexit__(exc_type, exc_val, exc_tb)
+
+  async def publish(self, message: PubSubMessage) -> PublishMetadata:
+    partition = self._routing_policy.route(message)
+    assert partition in self._publishers
+    return await self._publishers[partition].publish(message)