feat: add event handler webhook for certificate generation

[arslanashraf7]

This pull request introduces support for handling certificate creation webhooks when a learner passes a course in the Open edX LMS. The main changes add a new signal handler for the COURSE_GRADE_NOW_PASSED event, which triggers a Celery task to notify an external system to create a certificate. Configuration options and tests for the new functionality are also included.

Certificate Webhook Integration:

• Added a signal receiver (listen_for_passing_grade) for the COURSE_GRADE_NOW_PASSED event, which checks certificate eligibility and dispatches a webhook notification when a learner passes a course. (ol_openedx_events_handler/receivers/certificate_passing_receiver.py, [1] [2] [3] [4]
• Implemented a Celery task (create_certificate_for_passing_grade) to send a POST request to the configured certificate webhook endpoint, including error handling and retry logic. (ol_openedx_events_handler/tasks/certificate_passing.py, src/ol_openedx_events_handler/ol_openedx_events_handler/tasks/certificate_passing.pyR1-R80)
• Added utility function to validate certificate webhook configuration. (ol_openedx_events_handler/utils.py, src/ol_openedx_events_handler/ol_openedx_events_handler/utils.pyR37-R65)

Configuration and Documentation:

• Introduced new settings (CERTIFICATE_WEBHOOK_URL, CERTIFICATE_WEBHOOK_ACCESS_TOKEN) with support for environment token overrides, and documented their usage in the README. (ol_openedx_events_handler/settings/common.py, [1]; ol_openedx_events_handler/settings/production.py, [2]; README.rst, [3]
• Updated the changelog and project version to 0.2.0 to reflect the new functionality. (CHANGELOG.rst, [1]; pyproject.toml, [2]

Testing:

• Added unit tests for both the signal receiver and the Celery task to ensure correct behavior under various conditions, including eligibility checks and error scenarios. (tests/test_certificate_passing_receiver.py, [1]; tests/test_certificate_passing_tasks.py, [2]

These changes enable automated certificate creation workflows integrated with external systems upon course completion.### What are the relevant tickets?
https://github.com/mitodl/hq/issues/9693, https://github.com/mitodl/hq/issues/10267

Description (What does it do?)

• The PR adds a certificate event listener in edx instance
• The listener then calls the MIT app's certificate generation API to generate the certificates

Screenshots (if appropriate):

• Desktop screenshots
• Mobile width screenshots

How can this be tested?

NOTE: Will be tested along with the counterpart mitodl/mitxonline#3459

• Setup on this branch
• Install the plugin ol_openedx_events_handler
• Set up a course with a grading policy and units
• Enroll a user in the paid course mode of this course
• Set up the configuration values for CERTIFICATE_WEBHOOK_URL (This is the API URL of the API from MITx Online) and CERTIFICATE_WEBHOOK_ACCESS_TOKEN (This is the staff token from your MITx Online instance)
• Set up the course to be self-paced and pass the user in a course, the webhook should be called, and a certificate should be created in MITx Online if validation passes
• Set up the certificate display behavior to Immediately upon passing and pass the user in the course, the certificate webhook should be called, and a certificate should be created in MITx Online
• Set up certificate display behavior to any value other than Immediately upon passing, and the webhook should not be called.

Additional Context

Will be tested along with the counterpart mitodl/mitxonline#3459

[Anas12091101]

Tested E2E. LGTM 👍

Just a couple of small changes

[Anas12091101]

I believe that only the second point is valid here.

[Anas12091101]

enrollment_mode_for_user also returns an is_active flag. We should include an is_active check here to ensure certificates are not created for inactive enrollments.

Suggested change

	enrollment_mode, _ = CourseEnrollment.enrollment_mode_for_user(user, course_id)
	enrollment_mode, is_active = CourseEnrollment.enrollment_mode_for_user(user, course_id)
	if not is_active:
	return False

[arslanashraf7]

@Anas12091101 FYI, I've reduced the task retry count to 2, so the tasks will run a total of 3 times now. Once at start and twice at the retry. I've done this in e6fca7b. I'll merge the PR once the CI checks are green.