feat: Cache Bigtable client (feast-dev#3602)

* Cache Bigtable client

Signed-off-by: Yusuke Nishioka <yusuke.nishioka.0713@gmail.com>

* Run format

Signed-off-by: Yusuke Nishioka <yusuke.nishioka.0713@gmail.com>

---------

Signed-off-by: Yusuke Nishioka <yusuke.nishioka.0713@gmail.com>

Author: ysk24ok

sdk/python/feast/infra/online_stores/bigtable.py

@@ -335,4 +335,8 @@ def teardown(
     def _get_client(
         self, online_config: BigtableOnlineStoreConfig, admin: bool = False
     ):
-        return bigtable.Client(project=online_config.project_id, admin=admin)
+        if self._client is None:
+            self._client = bigtable.Client(
+                project=online_config.project_id, admin=admin
+            )
+        return self._client