Move source_objects datatype check out of GCSToBigQueryOperator.__init__

[josh-fell]

Closes: #20333

Currently there is a datatype check on source_objects within the GCSToBigQueryOperator.__init__() method. This is a template field for the operator. If a user tries to pass in an XCom value that will be rendered as a list (i.e. render_template_as_native_obj=True at the DAG level), the datatype check incorrectly assumes the value is a string and wraps the value in a list. When the XCom value is then rendered during execute() the value becomes a List[List] instead of the desired List type.

Moving this datatype check into the execute() method of the operator will allow any template rendering to occur before the check is performed.

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[potiuk]

Really nice find!

[github-actions]

The PR is likely OK to be merged with just subset of tests for default Python and Database versions without running the full matrix of tests, because it does not modify the core of Airflow. If the committers decide that the full tests matrix is needed, they will add the label 'full tests needed'. Then you should rebase to the latest main or amend the last commit of the PR, and push it with --force-with-lease.