Add ability to use Sepia search API

Hi.

First, thanks for this plugin!

Would it be possible to add an option to use Sepia Search API instead of the one from selected instances?

For more flexibility, that would be even better if we could specify a Search Index url and defaulting it to Sepia so we could use our own index instead of the Sepia's one (which is actually based on instances.joinpeertube.org)

Edit: I just saw it's possible to just add sepiasearch.org in the instances list and it searches on it.
So, I think it could be nice if we had a tickbox for enabling Sepia Search instead of having to add it manually. The tickbox could be opt out so we wouldn't have to configure anything when we install PeerTube Integration.

Edit²: Also, thumbnails are broken when retrieving content from Sepia Search:

Start by reviewing the PeerTube integration's existing instance-search flow and its thumbnail retrieval behavior. Clarify whether the change should support a Sepia opt-out checkbox, a configurable Search Index URL, or all requested behaviors, then verify the reported broken thumbnails. Done means the selected search source works and returned thumbnails display correctly.