search: add repo pager job
Created by: rvantonder
A polished up version of previous repo pagination job in https://github.com/sourcegraph/sourcegraph/pull/32029. The main change is that I realized using the mapper interface actually reduces the complexity and brittleness of more separate ad-hoc case statements/type assertions.
This after trying to add methods to the current Job
interface to populate repos, I realized that this is going to make everything incredibly ugly and more difficult to reason about.
Test plan
Added unit test for the main partitioning logic. This job isn't hooked up or used yet (no backend-integration
test needed).