feat: allow forcing cache control header

[albaad]

• What it solves: Adds the ability to explicitly enable Cache-Control headers even when GraphQL responses contain errors. This is useful for scenarios where certain error outcomes should be cacheable (e.g., 400/404 for missing or not-yet-available resources), allowing downstream caches/CDNs to respect custom max-age and scope.
• How: Introduces a helper cache.ContextWithForceCacheControl(ctx, true) that marks a request as eligible for Cache-Control header emission even when errors are present. The middleware uses this signal to compute and write the header from existing cache hints.
• Why: Some APIs benefit from caching specific error responses to reduce load and improve latency, particularly for predictable cases like resource not found or not ready yet.
• Docs: README updated with example usage showing how to enable forced cache control on error cases.