gcs_to_bq: Support schema in different bucket

[maroshmka]

Description

Schema path (schema_object) for table in GCSToBigQueryOperator is downloading schema from source bucket.

New feature should support support also pulling schemas from different bucket.

Use case / motivation

I guess there are lot of use-cases. To mention one concrete:

• we want to have schemas at 1 place, for us its terraform, where we define all resources and ACLs
• those schemas are exported to GCS by CI
• we want to use them in loads to BQ tables

In case there is a schema change, we make 1 change (in terraform) and it works. Otherwise we would need to do terraform change, apply, then either update schema_fields in operator and release it or to allow adding fields on table load (which we don't necessarily want all the time).

Related Issues

Not directly, but at least - #8280 - polish system tests to operator while at it

[maroshmka]

Implemented draft, if its ok to have this feature, I can finish the system tests :)