Support limit/offset in unified search

[juliusknorr]

As reported by @SystemKeeper the unified search providers seem to not properly support limiting and paginating the results.

I checked and the page content provider has some limit from the full text search index, but that is applied per collective, the other providers are just returning all results. We can probably do a quick fix to just skip results before the cursor and after the limit, but would still need to fetch most of the results in the backend. Otherwise we need to rethink the logic to collect search results.

Filing this issue to not forget

Talk PR for inspiration nextcloud/spreed#14024