feat: get_library_unit_usage_key in new content_libraries.units module [FC-0083]

[ChrisChV]

Description

• Refactored the code to use LibraryElementKey instead of LibraryCollectionKey
• New units module created in content_libraries.
• get_library_unit_usage_key func created.
• Which edX user roles will this change impact? "Developer"

Supporting information

• Depends on: feat: Create LibraryContainerLocator for Library Containers [FC-0083] opaque-keys#369
• Github issues:
  • What should library container usage keys look like? openedx-learning#282
  • Backend: implement opaque key format for structures in libraries frontend-app-authoring#1704
• Internal ticket: FAL-4108

Testing instructions

• Check all the code.
• Verify that the test covers the changes.

Deadline

None

Other information

• Bump opaque-keys version before merge this PR.

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[bradenmacdonald]

I have included most of these changes in #36371 so I don't think we need this PR. Thanks!