Add option to exclude system workers in ListWorkers API (#777)

## What
Add `bool include_system_workers` field to `ListWorkersRequest` proto.
When true, the response includes system workers that are not created by
the user. By default, system workers are excluded.

## Why
System workers (per-namespace workers for scheduler, batcher, etc.) show
up in ListWorkers results, which is confusing for users who only care
about their own workers.

## How did you test it?
Build

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: rkannan82