Returns a list of book tags for authors specified by a list of identifiers.

This method getTagsOfAuthorsBooks takes two parameters:

• authors — a list of Author objects.
• searchAuthorIds — a list of author IDs whose book tags need to be retrieved.

It returns a list of tags (List<Tag>) for all books by authors whose IDs are in searchAuthorIds.

Here's how it works:

1. Filters the list of authors, keeping only those whose id is in searchAuthorIds.
2. For each such author, gets a stream of their books.
3. For each book, gets a stream of tags.
4. Collects all tags into a single list.

Thus, the method efficiently extracts all tags of books by the specified authors.

Example usage:

List<Tag> tags = getTagsOfAuthorsBooks(allAuthors, Arrays.asList("author1", "author3"));

This will return all tags of books by authors with IDs "author1" and "author3".