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.

[landrade]

Hi @albaad,

Could you add an explanation of what this PR resolve in the description?

[albaad]

Hi @landrade,

I’ve updated the PR description with an explanation of what it resolves—sorry for the long delay! Let me know if you need anything else.

[aartal]

Hi @landrade,

Just checking if there’s anything missing or something I can do to help move this forward. Happy to address any feedback or make further changes if needed.

Thanks for your time!