feat: add deprecation warnings for .blob accessor and read_gbq_object_table (#2408)

This PR introduces ApiDeprecationWarning (which inherits from
FutureWarning) to the following APIs to notify users of their
  upcoming removal in a future release:

- `Series.blob` accessor: Added a warning to the property getter in
bigframes/series.py.
- `read_gbq_object_table`: Added a warning to the implementation in
bigframes/session/Session. This will also trigger when called via
bigframes.pandas.read_gbq_object_table.

Fixes #<478952368> 🦕

Author: shuoweil

bigframes/series.py

@@ -317,6 +317,14 @@ def list(self) -> lists.ListAccessor:
 
     @property
     def blob(self) -> blob.BlobAccessor:
+        """
+        Accessor for Blob operations.
+        """
+        warnings.warn(
+            "The blob accessor is deprecated and will be removed in a future release. Use bigframes.bigquery.obj functions instead.",
+            category=bfe.ApiDeprecationWarning,
+            stacklevel=2,
+        )
         return blob.BlobAccessor(self)
 
     @property

bigframes/session/__init__.py

@@ -2291,6 +2291,11 @@ def read_gbq_object_table(
             bigframes.pandas.DataFrame:
                 Result BigFrames DataFrame.
         """
+        warnings.warn(
+            "read_gbq_object_table is deprecated and will be removed in a future release. Use read_gbq with 'ref' column instead.",
+            category=bfe.ApiDeprecationWarning,
+            stacklevel=2,
+        )
         # TODO(garrettwu): switch to pseudocolumn when b/374988109 is done.
         table = self.bqclient.get_table(object_table)
         connection = table._properties["externalDataConfiguration"]["connectionId"]