Add onSkip hook for middleware extractor returning null

[thedevyashsaini]

Context The extractor can return null to skip tracking, but the only visibility is a log line. Add a hook so apps can track skip reasons.

Goal Expose an optional onSkip callback that is called when extractor returns null.

Acceptance Criteria

• middlewareEventConsumer config accepts optional onSkip.
• onSkip is called with { req, reason?: string } when extractor returns null.
• Default behavior unchanged (still skips tracking silently if callback not provided).

Notes

• Reason can be a short string like "extractor_returned_null" or based on caller-provided value.