feat(gzip): add option to combine default and custom compression decisions

[kekexiaoai]

…sions

This patch introduces a new option WithCombineDefaultAndCustom to the gin-contrib/gzip middleware, allowing developers to control how customShouldCompressFn interacts with the default shouldCompress logic.

Previously, when a custom compression decision function was provided, the default compression rules (Accept-Encoding, excluded paths, excluded extensions, Upgrade connections, etc.) were completely bypassed. This made it impossible to extend the default behavior without fully re-implementing it externally.

Changes introduced:

• Add combineDefaultAndCustom flag to the gzipHandler config.
• Update compression decision logic:
  • When combineDefaultAndCustom = true, compression is enabled only if: customShouldCompressFn(c) && shouldCompress(c.Request)
  • When combineDefaultAndCustom = false (default behavior), customShouldCompressFn fully overrides default logic.
• Add new option: gzip.WithCombineDefaultAndCustom()
• Preserve full backward compatibility.
• Update handler logic to use the new combined decision flow.

This enhancement allows developers to:

• Extend default compression behavior safely.
• Add additional custom filters without losing built-in protections.
• Support advanced use cases such as conditionally enabling gzip only on specific cluster nodes (e.g. leader-only compression) while still respecting default checks.