feat: add `edx.certificate.revoked` event

[MICROBA-1075]
- Adds a new `edx.certificate.revoked` event to the LMS.
- Refactor of our certificate revocation functions in the GeneratedCertificate model.

This new event will be emit when a GeneratedCertificate with the status of `downloadable` is revoked (through the `invalidate(), mark_notpassing(), or mark_unverified() functions). Event will have a `source` field that will allow us how our certificates are being revoked from learners and can be broken down in the following way:
*Invalidate*
- allowlist_removal
- certificate_generation
- certificate_regeneration
- certificate_service
- certificate_invalidation_list
- bulk_certificate_regeneration

*mark_notpassing*
- certificate_generation
- notpassing_signal

*unverified*
- certificate_generation