Book/Book-shelf block: Allow custom links on book

The Book block is currently very basic, showing an image and rudimentary metadata. However we mean to expand this, ISSN/ISBN is just one example, but lots of metadata could be valuable.

There's an open design task as to how this metadata is both added by an editor, and viewed by a visitor. One idea is that the same happens whether you click a book in a book shelf block, or just a book: it flips over on the back to show metadata and perhaps a link to a review of your choice.

Some important additions to keep ind for this design effort, though, comes from @melchoyce, who would like to see these use cases accommodated:

- Book recommendation lists, where someone curates a list of books into one post. In this case, I can see the bookshelf block being used to showcase all the books towards the top of the post, and each book would anchor link down to its review.
- Similarly, book recommendation pages that are common on “though leader” websites. I’d either expect clicking on a book to take you to a new page with details about the book, or to take you to a place to purchase the book (with perhaps an affiliate code embedded in the link).
- Authors showcasing all of their books on a page. In this case, I’d expect the book to link to a new page or open a modal with more details about the book: publisher, release date, synopsis, reviews, links to buy, etc.