search: separate symbol jobs for zoekt and searcher

Created by: rvantonder

This creates two separate jobs for symbol search: one that runs on the Zoekt backend, and one that runs on the searcher backend. This is the separation of the current symbol search job, which does both. Basically I'm doing the same thing for symbol search that I did for text search, so that we can page over symbol jobs that need paging. This is the first of 3 PRs, the third PR does the switch to use this: https://github.com/sourcegraph/sourcegraph/pull/32538. Everything was tested before I put the PRs up for review :-)

This is setup, it is not used yet.

Test plan

Unused currently.