feat: feature store integration base (Part I)

add private _featurestores module
add Featurestore/EntityType/Feature class
add featurestore_utils.py
add FeaturestoreClientWithOverride in utils __init__.py
update compat services and types to include featurestore and featurestore online
add get/update/delete/list/search methods
add unit tests for above methods
add skeleton for create/ingest/read/serve methods

google/cloud/aiplatform/_featurestores/__init__.py

@@ -0,0 +1,26 @@
+# -*- coding: utf-8 -*-
+
+# Copyright 2021 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.
+#
+
+from google.cloud.aiplatform._featurestores.featurestore import Featurestore
+from google.cloud.aiplatform._featurestores.entity_type import EntityType
+from google.cloud.aiplatform._featurestores.feature import Feature
+
+__all__ = (
+    "Featurestore",
+    "EntityType",
+    "Feature",
+)