Add test coverage for optional BigQuery dependency

[pankaj-bind]

PR Description

This update addresses the issue of limited test coverage for optional dependencies within the langchain-google-community package, using BigQueryLoader as the initial implementation.

Previously, modules with optional dependencies might not have been adequately tested for scenarios where those dependencies were missing, potentially leading to unexpected errors for users with minimal installations.

This change enhances the robustness of the package by:

1. Adding a Guarded Import: The BigQueryLoader now checks for the google-cloud-bigquery package upon instantiation. If the dependency is not found, it raises a clear ImportError with instructions on how to install it.
2. Introducing a New Unit Test: A dedicated unit test has been added to simulate an environment where google-cloud-bigquery is not installed. This test verifies that the BigQueryLoader raises the correct ImportError, ensuring this behavior is maintained and preventing future regressions.

This approach improves the reliability and user experience of the library, providing graceful failures and clear guidance when required dependencies are missing.

All existing and new tests pass successfully after these changes.