Add bigframes.bigquery.rand() function

This change adds `bigframes.bigquery.rand()` which wraps the BigQuery `RAND()` function.
It accepts a Series or DataFrame as input to determine the shape and index of the output Series.
It includes a warning about non-determinism in the docstring.

Additionally, `SqlScalarOp` has been updated to accept an `is_deterministic` argument, which is set to `False` for `rand()`.

Tests are added in `tests/unit/bigquery/test_mathematical.py` and `tests/system/small/bigquery/test_mathematical.py`.

Ran `nox -s format lint mypy` and confirmed no issues were introduced.

Co-authored-by: tswast <247555+tswast@users.noreply.github.com>

Author: google-labs-jules[bot]

bigframes/bigquery/_operations/mathematical.py

@@ -16,8 +16,7 @@
 
 from typing import Union
 
-from bigframes import dataframe
-from bigframes import dtypes
+from bigframes import dataframe, dtypes
 from bigframes import operations as ops
 from bigframes import series