[Feature] Support a proxy/custom endpoint for BigQuery queries

Description of Feature

A Cloud customer wants to route all GrowthBook-issued BigQuery queries through Alvin AI for query optimisation.

GrowthBook currently has no BigQuery equivalent to Snowflake’s SNOWFLAKE_PROXY, leaving cloud-hosted customers unable to configure this without moving to self-hosting.

When self hosting the workaround is to set the BIGQUERY_EMULATOR_HOST env var as described in this Alvin doc

We use the native bigquery nodejs package for the integration, so this should work.