Support Installed code via python callable string

[john-drews]

i want to be able to define a path to a custom function that i am installing into my airflow environment. This will allow me to test my dag locally without having to define a path and simply refer to the package path instead.

Example

        branch_by_bq_load_status:
            operator: airflow.operators.python.BranchPythonOperator
            python_callable: custom.path.to.func

[pankajastro]

@john-drews, Thank you very much for the contribution, could you please fix the CI issue?