API: endpoint for updating item body

[mathisdt]

Some feeds I read have a lot of ads in them. I'd like to have a possibility to cut that part out of the body, so after creating this small Python script for automatically marking items as read, I thought that it might be a good idea to be able to update the body (not id, guid or guidHash) via a POST request.

The other possibility would of course be to subscribe the feed through a proxy, which would filter out unwanted content before Nextcloud News sees it. But this looks like even more work, so I like the update endpoint better...

[SMillerDev]

If you can find a good way to keep track of the body so that it doesn't always overwrite on upstream updates I'll happily review a pull request.