codeintel: Add dbstore method SelectRepositoriesForIndexScan

Created by: efritz

This PR introduces a dbstore method SelectRepositoriesForIndexScan that returns a batch of repositories for which to schedule auto-indexing jobs. This is similar to what we did for data retention.

Open question: How do we sub-select from repositories to ensure we don't scan the Cloud repos table? We need to have some hard-coded way to limit the number of repositories here, but that's how global policies work. How do we artificially restrict this set?