Implement feature to retrieve and save all books from user's playlists

[sweep-ai]

Description

This PR implements a new feature to retrieve and save all the books from the user's playlists in the Safari Books Online library. It adds the functionality to read the user's playlists, retrieve the list of books in each playlist, and download each book.

Summary of Changes

• Added a new function to retrieve the user's playlists by making a request to the appropriate API endpoint and parsing the response.
• For each playlist, retrieved the list of books by making a request to the appropriate API endpoint and parsing the response.
• Downloaded each book by calling the existing function to download a book with the book ID as an argument.

These changes enable users to easily retrieve and save all the books from their playlists, providing a convenient way to access their favorite books offline.

Fixes #1.

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To checkout this PR branch, run the following command in your terminal:

git checkout sweep/retrieve-save-books-from-playlists

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