Add config generate-sample command to App Proxy Coordinator and Worker

## Problem

Other Backend.AI components (manager, agent, storage, webserver) provide a `config generate-sample` CLI command to generate sample configuration files. However, App Proxy Coordinator and Worker components are missing this command.

## Expected Behavior

Users should be able to generate sample configuration files for both components:

```bash
./backend.ai app-proxy-coordinator config generate-sample
./backend.ai app-proxy-worker config generate-sample
```

## Scope

- Add `config generate-sample` command to App Proxy Coordinator CLI
- Add `config generate-sample` command to App Proxy Worker CLI
- Follow the same pattern as other components (manager, agent, storage, webserver)
- Ensure generated sample configs include all necessary fields with appropriate defaults and comments

## References

Existing implementations:

- Manager: `src/ai/backend/manager/cli/`
- Agent: `src/ai/backend/agent/cli/`
- Storage: `src/ai/backend/storage/cli/`
- Webserver: `src/ai/backend/web/cli/`

JIRA Issue: BA-3052