Respect HTTP Cache-Control header when caching an Excel Add-in's JSON Metadata file

Currently, Excel will permanently cache an Add-in's custom functions JSON metadata file. This happens when the Add-in is first added to Excel.

The JSON metadata file may be updated from time to time for various reasons. It could be as trivial as correcting a typo in a custom function's description. It could be as serious as adding a new custom function altogether. Regardless, Excel currently doesn't fetch the new JSON file.

The only ways to get Excel to fetch the new JSON file appear to be:
(1) Submit a new XML manifest to AppSource with a new version number, even though the rest of the manifest are unchanged. This also contradicts the AppSource submission FAQ which says we don't have to resubmit if we make changes only to the web service.
(2) Ask users to manually reinsert the Add-in from Office Store.
(3) Ask users to manually clear the Office Cache.

All of the above seems unnecessarily complicated, when Excel could simply respect the HTTP Cache-Control headers returned by the server.

Incidentally, Excel already respects the HTTP Cache Control headers for other files, such as the custom functions HTML and JavaScript files. So I suggest we extend that same treatment to the custom functions JSON metadata file.