Store pre-aggregations in a directory within an export bucket with the BigQuery driver

Is your feature request related to a problem? Please describe. I want to store pre-aggregations for different tenants in separate folders within the same export bucket.

Describe the solution you'd like Maybe to have CUBEJS_DB_EXPORT_BUCKET changed to CUBEJS_DB_EXPORT_PATH, or at least have the semantic changes to support both plain buckets and s3://... or gs://... paths like boto/gsutil does.

Describe alternatives you've considered There is no alternative solution except for using different buckets altogether.

Additional context The BigQuery driver uses a bucket eg CUBEJS_DB_EXPORT_BUCKET=cube-cloud-staging-export-bucket. The Athena driver supports S3 paths eg CUBEJS_DB_EXPORT_BUCKET=s3://cubejs-benchmarks/home/app/.../.