Implement config diff CLI Command

## Overview

Implement `config diff` CLI command that compares configurations between different layers or environments.

## Requirements

- Compare config between layers (file vs etcd, file vs merged)
- Compare config between environments (dev vs prod etcd)
- Highlight differences with added/removed/changed values
- Support filtering by config section

## Use Cases

1. See what values etcd overrides from file config
1. Compare production vs development configurations
1. Identify config drift between environments

## Acceptance Criteria

- [ ] Compares file config vs etcd config
- [ ] Compares file config vs merged config
- [ ] Shows added/removed/changed values clearly
- [ ] Supports --format option for output
- [ ] Works for all components

JIRA Issue: BA-3474