feat: Loader updates for custom presentations

[AfaqShuaib09]

PROD-4007

This PR carries CSV loader updates to handle b2b-custom representations. Now the CSV loader can handle the creation/update of multiple variants in a single ingestion. This also handles loader ingestion report email updates for multiple variants.

Testing Instructions:

• Download the dummy json file from the given link
• Place this in course-discovery root folder.
• Run populate_executive_education_data_csv command

  ./manage.py populate_executive_education_data_csv --product_source=2u --output_csv=output.csv --dev_input_json=dev.json --use_getsmarter_api_client=True

• Then, finally, run the import_course_metadata command:

  ./manage.py import_course_metadata --partner_code=edx --csv_path=output.csv --product_type=EXECUTIVE_EDUCATION --product_source=2u

(Make sure to update the slug format dict in base.py; otherwise, it might raise an error.)