feat: entity link view and api

[navinkarkera]

Description:

Adds api for listing upstream entity links for a given course.

Test instructions: See openedx/frontend-app-authoring#1641

Part of: openedx/frontend-app-authoring#1565
Private-ref: FAL-4006

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[pomegranited]

👍 Working beautifully.

• I tested this using the test instructions from feat: course libraries page frontend-app-authoring#1641
  Also had to run migrations, recreate_upstream_links and reindex_studio to get the data ready to test.
• I read through the code that's changed since feat: upstream-downstream entity linking #36111 (ref compare)
• I checked for accessibility issues N/A
• Includes documentation - code comments
• User-facing strings are extracted for translation N/A