wip search config endpoint for multiple branches
Created by: keegancsmith
This is a WIP commit illustrating how we will pass down which branches to index for a repository to Zoekt. It is only documenting which fields we need to send.
Note: zoekt-sourcegraph-indexserver doesn't yet support this. If you want you can also update zoekt-sourcegraph-indexserver to support this. I would suggest that if Branches is non-empty zoekt-sourcegraph-indexserver would use zoekt-git-index instead of zoekt-archive-index. This would be a nice way to feature flag.
Part of https://github.com/sourcegraph/sourcegraph/issues/6728