feat: BulkWriter implementation (#384)

* feat: added `write` method to batch classes

* added docstrings to all 3 batch classes

instead of just the base

* updated batch classes to remove control flag

now branches logic via subclasses

* fixed broken tests off abstract class

* fixed docstring

* refactored BulkWriteBatch

this commit increases the distance between WriteBatch and BulkWriteBatch

* began adding [Async]BulkWriter

* continued implementation

* working impl or BW

* tidied up BW impl

* beginning of unit tests for BW

* fixed merge problem

* initial set of BW unit tests

* refactored bulkwriter sending mechanism

now consumes off the queue and schedules on the main thread, only going async to actually send

* final CI touch ups

* 🦉 Updates from OwlBot

See https://github.com/googleapis/repo-automation-bots/blob/master/packages/owl-bot/README.md

* 🦉 Updates from OwlBot

See https://github.com/googleapis/repo-automation-bots/blob/master/packages/owl-bot/README.md

* moved BulkWriter parameters to options format

* rebased off master

* test fixes

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>

google/cloud/firestore_v1/async_client.py

@@ -43,13 +43,18 @@
     DocumentSnapshot,
 )
 from google.cloud.firestore_v1.async_transaction import AsyncTransaction
+from google.cloud.firestore_v1.bulk_writer import (
+    BulkWriter,
+    BulkWriterOptions,
+    SendMode,
+)
 from google.cloud.firestore_v1.services.firestore import (
     async_client as firestore_client,
 )
 from google.cloud.firestore_v1.services.firestore.transports import (
     grpc_asyncio as firestore_grpc_transport,
 )
-from typing import Any, AsyncGenerator, Iterable, List
+from typing import Any, AsyncGenerator, Iterable, List, Optional
 
 
 class AsyncClient(BaseClient):
@@ -287,6 +292,16 @@ async def collections(
         async for collection_id in iterator:
             yield self.collection(collection_id)
 
+    def bulk_writer(self, send_mode: Optional[SendMode] = None) -> BulkWriter:
+        """Get a BulkWriter instance from this client.
+
+        Returns:
+            :class:`@google.cloud.firestore_v1.bulk_writer.BulkWriter`:
+            A utility to efficiently create and save many `AsyncWriteBatch` instances
+            to the server.
+        """
+        return BulkWriter(client=self, options=BulkWriterOptions(mode=send_mode))
+
     def batch(self) -> AsyncWriteBatch:
         """Get a batch instance from this client.