feat: apply ADRs standardization to enrollment apis

[taimoor-ahmed-1]

Description

Introduces /api/enrollment/v2/ as a parallel new version of the enrollment API, applying the FC-0118 ADRs from the start. v1 endpoints at /api/enrollment/v1/ are unchanged — clients migrate at their own pace.

What's new
openedx/core/djangoapps/enrollments/v2/ — viewsets, service layer, paginator, forms, serializers, tests
EnrollmentViewSet consolidates v1's EnrollmentListView + UnenrollmentView + EnrollmentAllowedView into a single router-registered ViewSet (actions: list, create, unenroll, allowed)
EnrollmentRetrieveView, UserRolesView, CourseEnrollmentDetailView, EnrollmentsAdminListView for endpoints with URL shapes that don't fit a router
EnrollmentOperationsService centralizes business logic and raises DRF exceptions (modern ADR 0029 pattern)
StandardizedErrorMixin promoted from cms/.../rest_api/mixins.py to openedx/core/lib/api/mixins.py so non-contentstore apps can use it

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