feat: add bronze-to-silver BigQuery template

[mohin-io]

Summary

• add a bronze-to-silver BigQuery template that ingests Frankfurter FX rates via ingestr and curates a silver layer with rolling metrics
• document configuration, quality checks, and usage in both the template README and the public templates docs
• provide starter .bruin.yml defaults and BigQuery scheduling metadata aligned with existing template conventions

Testing

• not run (template requires user-provided BigQuery project and datasets)

Closes #1220

[mohin-io]

Please create bronze and silver datasets in your BigQuery project (or update the asset names) before running the bruin run.
Or optionally expand the template with additional silver/gold assets once the baseline pipeline is validated.