feat(experimental): add AsyncAbstractObjectStream (#1546)

* add AsyncAbstractObjectStream

this will be the parent class for AsyncReadObjectStream and AsyncWriteObjectStream

* keep _AsyncAbstractObjectStream private

* include handle in tests

* remove unit tests for abstract class

* bucket_name and object_name cannot be NONE

* minor edit - add bidi-stream in doc string

Author: chandra-siri

google/cloud/storage/_experimental/asyncio/async_abstract_object_stream.py

@@ -0,0 +1,67 @@
+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import abc
+from typing import Any, Optional
+
+
+class _AsyncAbstractObjectStream(abc.ABC):
+    """Abstract base class to represent gRPC bidi-stream for GCS ``Object``.
+
+    Concrete implementation of this class could be ``_AsyncReadObjectStream``
+    or ``_AsyncWriteObjectStream``.
+
+    :type bucket_name: str
+    :param bucket_name: (Optional) The name of the bucket containing the object.
+
+    :type object_name: str
+    :param object_name: (Optional) The name of the object.
+
+    :type generation_number: int
+    :param generation_number: (Optional) If present, selects a specific revision of
+                              this object.
+
+    :type handle: bytes
+    :param handle: (Optional) The handle for the object, could be read_handle or
+                   write_handle, based on how the stream is used.
+    """
+
+    def __init__(
+        self,
+        bucket_name: str,
+        object_name: str,
+        generation_number: Optional[int] = None,
+        handle: Optional[bytes] = None,
+    ) -> None:
+        super().__init__()
+        self.bucket_name: str = bucket_name
+        self.object_name: str = object_name
+        self.generation_number: Optional[int] = generation_number
+        self.handle: Optional[bytes] = handle
+
+    @abc.abstractmethod
+    async def open(self) -> None:
+        pass
+
+    @abc.abstractmethod
+    async def close(self) -> None:
+        pass
+
+    @abc.abstractmethod
+    async def send(self, protobuf: Any) -> None:
+        pass
+
+    @abc.abstractmethod
+    async def recv(self) -> Any:
+        pass