add cache control headers to partials.json

[RayBB]

This was quite useful; would you mind creating a new issue to add the headings to partials.json ? That's another one which I think would work well with some browser-side caching! CC @jimchamp

Originally posted by @cdrini in #8927 (comment)

[cdrini]

After some more thinking I realized that unlike the /cdn endpoint which is hit on every pageload on openlibrary, this endpoint is different on every page; so having a browser-side cache would likely not be a huge improvement. Closing as not worth the potential benefit right now.