poc: Worker service for consolidating background tasks

Created by: efritz

This PR introduces a new worker service. Currently this service replaces background jobs related to codeintel that used to occur in each frontend. Additional background services will make its way into this service.

Fixes #18253 (closed).

Related changes:

• deploy-sourcegraph
• deploy-sourcegraph-docker
• deploy-sourcegraph-dot-com

Update: Continuing in:

• https://github.com/sourcegraph/sourcegraph/pull/21768
• https://github.com/sourcegraph/sourcegraph/pull/21774