feat: upgrade send_email api to drf ( 15th )

[awais786]

Issue

Steps to verify postman
You need to enable this flag http://local.edly.io:8000/admin/bulk_email/bulkemailflag/add/

permissions = required course role = staff or limited_staff

Try this URL

form-data as post

send-to = ["myself"] or ["myself","learners","staff"] or any cohort user e.g ["cohort:awa"] ( it has multiple scenarios )
subject= This is subject.
message=This is my message.

schedule=2024-08-29T06:01:00.000Z ( This param is optional )

---

Try this from instructor dashboard
Go to this URL

[awais786]

I'm not handling validation into serializer because there are multiple input combinations, and moving the validation into the serializer would require refactoring, which could introduce errors.

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX staging environment in preparation for a release to production.

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX production environment.