Make flux check optional in FlowETL dags

[jc-harrison]

There are some situations where the flux check task in a FlowETL dag is unnecessary (e.g. a two-stage ingestion process where data from a multiple-day file are ingested into a staging table, and then daily dag runs extract from that staging table - a flux check is needed in the first stage, but the second-stage daily runs do not need to check for flux again). Since the table flux check can be quite slow, create_dag should provide an option to disable this check.