feat: add catalog courses service and update policies

[Ang-m4]

Description

This pull request adds all the necessary logic to add and remove courses from a catalog.
It introduces a new catalog_course service that handles course position ordering, bulk additions, and bulk removals of courses in a catalog.

Key changes

• Added two new endpoints to manage catalog courses:
  • POST /manage/catalogs/<id>/add_courses
  • POST /manage/catalogs/<id>/remove_courses
• Created two new schemas to document how to use both endpoints.
• Added the necessary policy checks to validate that new catalog course additions are valid, complying with the organization logic and base catalog logic.
• Removed the POST (create) catalog course from the CatalogCourseEnrollmentView, since all catalog course additions will now go through the /add_courses endpoint.
• Updated the PATCH/PUT endpoint at /manage/catalogs/<catalog_id>/courses/<course_id> to allow updating the position of a single course within a catalog.

How to test

1. Go to the Swagger UI http://local.openedx.io:8000/partner_catalog/swagger
2. Call POST /manage/catalogs/<id>/add_courses with a list of valid course IDs that belong to the catalog’s organization.
   • Verify in the response that the courses are now listed under that catalog.
3. Try adding a course that does not belong to the catalog’s organization:
   • The request should fail with a validation/policy error.
4. Call POST /manage/catalogs/<id>/remove_courses with one or more course IDs already associated.
   • Verify that those courses are removed from the catalog in the response.
5. Use PATCH or PUT on /manage/catalogs/<catalog_id>/courses/<course_id>:
   • Update the position field of a course.
   • Verify that the new position is reflected in the response.