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` and `ObjGetAccessUrlWithDuration` operations in `bigframes/operations/blob_ops.py`.
- 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`.

Author: google-labs-jules[bot]

bigframes/bigquery/__init__.py

@@ -18,7 +18,7 @@
 
 import sys
 
-from bigframes.bigquery import ai, ml
+from bigframes.bigquery import ai, ml, obj
 from bigframes.bigquery._operations.approx_agg import approx_top_count
 from bigframes.bigquery._operations.array import (
     array_agg,
@@ -158,4 +158,5 @@
     # Modules / SQL namespaces
     "ai",
     "ml",
+    "obj",
 ]