feat: Onboard bigquery-data-analytics to Evalbench CI pipeline

[omkargaikwad23]

This PR adds the necessary configuration files to enable automated evaluations for the bigquery-data-analytics extension repository using the evalbench framework whenever release is triggered. This setup is modeled after the existing pipeline in the cloud-sql-postgresql repository.

Key Changes

cloudbuild.yaml:

Added to the root directory to handle the evaluation workflow on release branches. It fetches PR data and triggers the standalone evaluation.

evals/ directory:

Created with all necessary configurations:

1. ci_metadata.yaml: Tracks the extension ID and release version.
2. dataset.json: Contains sample BigQuery-specific evaluation scenarios (bq-search-catalog and bq-ask-insights).
3. gemini_2.5_pro_model.yaml: Configures the Gemini 2.5 Pro model for scoring and simulation.
4. model_config.yaml: Configures the environment and setup for the BigQuery extension.
5. run_config.yaml: The main experiment configuration file for Evalbench.
6. substitute_env.py: A helper script to substitute environment variables in the YAML configs.

[prernakakkar-google]

Fix lint checks