refactor: convert content libraries python API from attr.s to dataclass, fix types [FC-0083]

[bradenmacdonald]

(Separated from #36371 to reduce the diff)

Description

The functionality of the third-party attrs library is now available in the python standard library, and the syntax of the built-in version is much nicer. This PR updates the content libraries API to use the standard version, and adds a bunch of missing type hints.

Technically, this is a small breaking change to the content libraries API:

• The learning_package field on ContentLibraryMetadata objects was replaced with learning_package_id. I'm not even sure if our libraries API should be exposing the learning package at all, but if we are we should only expose the ID.
• The objects returned from the various API methods are now immutable (technically not perfectly immutable but close enough). One test in the search app had to be updated but it's much cleaner now than it was before.

As the whole content libraries feature is in beta/development, I don't think anyone will be impacted by these very minor API tweaks.

Private ref: FAL-4052

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[pomegranited]

👍 So clean! Thank you @bradenmacdonald

• I tested this on my tutor dev stack by doing basic libraries operations in the Authoring MFE
• I read through the code
• I checked for accessibility issues N/A
• Includes documentation N/A
• User-facing strings are extracted for translation N/A

[bradenmacdonald]

Thanks @pomegranited ! Would you be able to merge this on Monday? It's very low risk but I don't wanna do a Friday afternoon merge.

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX staging environment in preparation for a release to production.

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX production environment.

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX production environment.