feat: Add BigQuery ObjectRef functions to bigframes.bigquery.obj

This change introduces support for BigQuery ObjectRef functions:
- `OBJ.FETCH_METADATA`
- `OBJ.GET_ACCESS_URL`
- `OBJ.MAKE_REF`

These are exposed via a new `bigframes.bigquery.obj` module.

Changes:
- Added `ObjMakeRefJson` operation in `bigframes/operations/blob_ops.py`.
- Updated `ObjGetAccessUrl` operation to support optional duration.
- Updated `bigframes/operations/__init__.py` to export new operations.
- Updated `bigframes/core/compile/ibis_compiler/scalar_op_registry.py` and `bigframes/core/compile/sqlglot/expressions/blob_ops.py` to support new operations.
- Created `bigframes/bigquery/_operations/obj.py` with the implementation of `fetch_metadata`, `get_access_url`, and `make_ref`.
- Created `bigframes/bigquery/obj.py` to expose the functions.
- Exposed `obj` module in `bigframes/bigquery/__init__.py`.
- Added unit tests in `tests/unit/bigquery/test_obj.py`.
- Updated `tests/unit/core/compile/sqlglot/expressions/test_blob_ops.py` snapshots.
- Fixed type checking and linting errors.

Author: google-labs-jules[bot]

tests/unit/bigquery/test_obj.py

@@ -13,13 +13,15 @@
 # limitations under the License.
 
 import datetime
+from unittest.mock import MagicMock
+
 import pytest
-from unittest.mock import MagicMock, patch
 
 import bigframes.bigquery.obj as obj
 import bigframes.operations as ops
 import bigframes.series as series
 
+
 def test_fetch_metadata_op_structure():
     op = ops.obj_fetch_metadata_op
     assert op.name == "obj_fetch_metadata"

tests/unit/core/compile/sqlglot/expressions/snapshots/test_blob_ops/test_obj_get_access_url/out.sql

@@ -11,7 +11,7 @@ WITH `bfcte_0` AS (
 ), `bfcte_2` AS (
   SELECT
     *,
-    OBJ.GET_ACCESS_URL(`bfcol_4`) AS `bfcol_7`
+    OBJ.GET_ACCESS_URL(`bfcol_4`, 'R') AS `bfcol_7`
   FROM `bfcte_1`
 ), `bfcte_3` AS (
   SELECT