Implement config validate CLI Command

## Overview

Implement `config validate` CLI command that validates configuration against the Pydantic schema.

## Requirements

- Validate file config against schema
- Validate etcd config against schema
- Validate merged config against schema
- Report validation errors with field paths and error messages
- Support validation of partial configs (etcd overrides)

## Use Cases

1. Validate config file before deployment
1. Check etcd config values are valid
1. Pre-flight check before starting component

## Acceptance Criteria

- [ ] Validates against Pydantic config schema
- [ ] Reports detailed error messages with field paths
- [ ] Exit code 0 on success, non-zero on validation failure
- [ ] Supports --source option (file, etcd, merged)
- [ ] Works for all components

JIRA Issue: BA-3475