Syntactic indexing: create job scheduling worker

[keynmol]

Depends on #59794

As part of this we should make some of the existing codeintel scheduling workers more generic: https://github.com/sourcegraph/sourcegraph/blob/main/cmd/worker/shared/main.go#L106-L118

The main purpose of this worker is to periodically schedule the actual indexing jobs for repositories and revisions that are not up-to-date (where "up-to-date" determination logic can be reused from the auto-indexing code).

As part of this issue, we should investigate how feasible it is to reuse the existing indexing policies, with the addition of Syntactic option - this way we don't duplicate configuration, and improvements to scheduling/repo matching mechanism will benefit both precise and syntactic.

A working end-to-end POC for this can be found in this branch: https://github.com/sourcegraph/sourcegraph/compare/main...syntactic-indexing-e2e#diff-1f01a15243f70514364308363d577121f701bfb20ca222a40f64d4e4735074de