Run the worker locally in a one-shot mode

In debusine we've automated wrapping many QA tasks in Debian. This is potentially usable to users.

For Debusine developers, it would be handy to be able to run a single task locally while developing it or working on a bug-fix.

The worker doesn't need any database access, just input artifacts and configuration for a task. These could be provided locally, or it could even use client-credentials to fetch artifacts from a debusine server.