Enable other authentication options for the BigQuery Syncer

[dorvak]

Complication: As of right now, the BigQuery Syncer only allows a Service-Account Key based authentication ("credentials.json"-File), which can be considered somewhat insecure and might be restricted to be used in some organisations.

Instead, other authentication options such as Application Default Credentials could be easily enabled, as the underlying SQLAlchemy BigQuery Library and the helper-method which is eventually using the google-Auth libraries (and the default-method etc. pp) are anyway being used in the cs_tools library.

Idea: Open up the interface/signature of the Syncer for BigQuery and allow passing other parameters (or none at all) to enable other authentication options.