refactor: downstream entity links api

[navinkarkera]

Description

Refactors downstream links API to handle multiple filters using a single API. Also adds a new route to return summary of library links for a given course.

Useful information to include:

• Which edX user roles will this change impact? Common user roles are "Course Author" and "Developer".

Supporting information

• Part of: Course: Libraries Page: Review Content Updates Tab frontend-app-authoring#1566
• Private-ref: FAL-4007

Also addresses following comments

• feat: entity link view and api #36190 (comment)
• feat: add API to return list of downstream blocks for an upstream [FC-0076] #36253 (comment)

Testing instructions

TBD

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