Add optional response caching support

[pgatzka]

User Story

As a developer, I want to cache GraphQL responses so that I can reduce latency and server load.

Description

Add optional caching layer for GraphQL responses:

Features:

• Cache strategy configuration (query-based, TTL-based)
• TTL settings per operation
• Cache invalidation hooks
• Cache metrics

Implementation options:

• In-memory cache (Caffeine)
• External cache support (Redis, etc.)

Acceptance Criteria

• Caching implementation added
• Cache configuration properties
• Cache metrics exposed via Micrometer
• Cache eviction strategies supported
• Documentation includes caching guide
• Implemented code is covered with automated tests
• Documentation has been updated