What are the relevant tickets?
What's this PR do?
Introduces a REST API for the certificates.
Where should the reviewer start?
How should this be manually tested?
Create a certificate for a user for a course and, using the url `/api/certificates/v0/certificates/<username>/<course_id>`, check that only that user can see it.
A (fake) certificate can be created using the `GeneratedCertificateFactory`: see
`lms/djangoapps/certificates/apis/v0/tests/test_views.py` for an example.
Any background context you want to provide?
It has been agreed that this API will be deprecated as soon as the [credentials](https://openedx.atlassian.net/wiki/display/AC/Credentials) will be deployed and the existent certificates will be migrated.
For more info please read edx#11821
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