Define common config module structure and base implementation

[HyeockJinKim]

Motivation

Establish the foundation for centralized UnifiedConfig infrastructure in the common module to eliminate code duplication across components.

Required Features

• Create directory structure under src/ai/backend/common/configs/unified/
• Define base classes and protocols for config schemas
• Create shared utilities for config loading (TOML, environment variables, etcd)
• Implement common validation and conversion patterns

Impact

• src/ai/backend/common/configs/unified/__init__.py
• src/ai/backend/common/configs/unified/base.py
• src/ai/backend/common/configs/unified/utils.py

Testing Scenarios

• Base infrastructure is in place
• Existing BaseConfigSchema is extended or adapted
• Documentation for the pattern is added

JIRA Issue: BA-3767