feat: add authz permission for certificates

[dwong2708]

Resolves: openedx/openedx-authz#199

Description

This PR introduces a new authorization permission for certificates and integrates it into the certificates endpoint behind a feature flag.

The goal is to align certificate access with the new AuthZ framework while maintaining backward compatibility through the flag.

Changes

• Implement courses.manage_certificates permission
• Add permission checks to the certificates endpoint handler (guarded by feature flag)
• Extend test coverage:
  - Validate access for course staff
  - Validate access for course editor (non-staff)
• Introduce a reusable test mixin for AuthZ certificate scenarios

Endpoint Affected

GET /api/contentstore/v1/certificates/<course_id>/

Now enforces courses.manage_certificates when the feature flag is enabled

Testing instructions

Please provide detailed step-by-step instructions for testing this change.

Deadline

Verawood

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[rodmgwgu]

@dwong2708 Please fill in the testing instructions in the description

[rodmgwgu]

Just a couple of small details, thanks!

[rodmgwgu]

I think to keep the previous behavior this should be LegacyAuthoringPermission.WRITE (was has_studio_write_access)

[rodmgwgu]

Didn't we already have a Mixin for this? (from your previous PR)