search: productionize hybrid zoekt+searcher prototype

Created by: keegancsmith

https://github.com/sourcegraph/sourcegraph/issues/33656 is shipped and running in production. Lets get this over the finish line and have it useable for the next release via a feature flag.

Tasks:

heuristics for falling back to normal searcher
- test on old commits to determine thresholds
- see if we need better caching behaviour by doing repeated searches on somewhat old commits
A dashboard showing performance and health of this feature
Observability to follow-up on how this is working in production
A way to disable once we default to on
stream from zoekt

Next branch cut is likely this week so unsure we will have enough data by then. So aiming to make this easy to opt into for admins.