Consider a common/unified hybrid search query API

Once we have both a common vector search story (#36350) and a common full-text search story (#37291), we can provide a common hybrid search story on top of these two. These would mean accepting two or more searches (vector, full-text), and combining the results via e.g. RRF (the actual implementation would be provider-specific, of course).

As with a common full-text search story, we are currently considering this as a stretch goal for 11.