GitHub App 설치 토큰: 요청별 재정의 헤더 도입

GitHub introduced a temporary header to support the gradual migration of installation tokens to a new stateless JWT format, allowing developers to test and validate compatibility before server-side rollout. This change improves security and token management while maintaining backward compatibility, but requires updates to token validation logic and database schemas. Developers must prepare their applications now to ensure seamless operation during the transition period.