test: Add unit testing using local polars-based engine (#1007)

Author: TrevorBergeron

bigframes/core/compile/polars/__init__.py

@@ -0,0 +1,25 @@
+# Copyright 2024 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+from __future__ import annotations
+
+import warnings
+
+try:
+    import polars  # noqa
+
+    from bigframes.core.compile.polars.compiler import PolarsCompiler
+
+    __all__ = ["PolarsCompiler"]
+except Exception:
+    warnings.warn("Polars compiler not available as polars is not installed.")