fix: use bigquery_client for gcs_stage methods

[tomasvotava]

When using gcs_stage method, self.client attribute of BaseBigQuerySink is actually a storage.Client instance, not bigquery.Client instance. This makes the clean_up fail if overwrite or upsert is used, because storage.Client has no query
method.

The solution here was to create a bigquery.Client using the existing factory when needed.

The downside of this is that the bq client may actually be needed elsewhere, also it could be reused but instead
we create a new instance for each table.

I'll let you consider whether to pull or close and consider this a hotfix.

Closes #69, IMO.

[tomasvotava]

I believe the checks are failing because of secrets access. If that's not the case, let me know and I'll fix things.

[z3z1ma]

makes sense, lgtm!