chore: move _read_gbq_colab to top-level bigframes.pandas (#1835)

* chore: move `_read_gbq_colab` to top-level `bigframes.pandas`

* 🦉 Updates from OwlBot post-processor

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

---------

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

Author: tswast

bigframes/pandas/__init__.py

@@ -38,6 +38,7 @@
 import bigframes.functions._utils as bff_utils
 from bigframes.pandas.core.api import to_timedelta
 from bigframes.pandas.io.api import (
+    _read_gbq_colab,
     from_glob_path,
     read_csv,
     read_gbq,
@@ -335,6 +336,7 @@ def reset_session():
     qcut,
     read_csv,
     read_gbq,
+    _read_gbq_colab,
     read_gbq_function,
     read_gbq_model,
     read_gbq_object_table,