Config Management CLI Tools for Backend.AI Components

## Overview

CLI tools for managing Backend.AI component configurations with layer visibility, validation, and migration support.

## CLI Commands

1. **config show** - Display current config with source layer info (file, env, etcd)
1. **config diff** - Compare configs between layers/environments
1. **config validate** - Validate config against schema
1. **config schema** - Display config schema with field descriptions
1. **config set/get** - Schema-aware config field manipulation
1. **config migrate** - Migrate from file/legacy etcd to new etcd paths

## Key Features

- Layer visibility showing which loader each config value originates from
- Schema-aware validation for all operations
- Safe migration from legacy config locations

## Dependencies

- BA-3459: Unified Config Infrastructure for All Backend.AI Components

JIRA Issue: BA-3472