feat: add Spanner first-party toolset (breaking change to BigQueryTool, consolidating into generic GoogleTool)

Spanner toolset support basic operations to interact with Spanner table metadata and query results.

Consolidate BigQueryTool into generic GoogleTool, so that BigQueryToolset and SpannerToolset can share.

PiperOrigin-RevId: 794259782

tests/unittests/tools/spanner/test_spanner_tool_settings.py

@@ -0,0 +1,27 @@
+# Copyright 2025 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
+
+from google.adk.tools.spanner.settings import SpannerToolSettings
+import pytest
+
+
+def test_spanner_tool_settings_experimental_warning():
+  """Test SpannerToolSettings experimental warning."""
+  with pytest.warns(
+      UserWarning,
+      match="Tool settings defaults may have breaking change in the future.",
+  ):
+    SpannerToolSettings()