add fetching thumbnails as part of useFeed

[estebanabaroa]

some websites dont have cors and thumbnails can be fetched

also using capacitor plugins and electron we can fetch thumbnails without cors

but we can't really fetch them async because it would displace the feed UI

so we would need some kind of API like PlebbitReactHooks.setFetch(fetch: Function) or useFeed({fetchThumbnail: Function}) where the frontend can inject a fetch function that doesnt have cors.

also useFeed would need to implement something to fetch the thumbnail URL aync of posts that are buffering, and add them to the posts, while not touching the posts that are already loaded, since we dont want to displace the UI.

since this only affect mobile and desktop, and not web, and fetching thumbnails is not that critical, and that it's pretty complicated, it is low priority.