Add support for adopting Cloudflare Ray-ID as correlation ID

Introduces WithAdoptCfRayHeader option to allow HTTP handlers to adopt incoming Cf-Ray headers as correlation IDs. When enabled, the handler will prefer Cf-Ray over X-Request-ID for correlation ID propagation.

This is particularly useful for services behind Cloudflare where the Ray-ID provides a unique request identifier across the CDN.

This is the first part of gitlab-com/gl-infra/production-engineering#27565 (comment 2741955770). Once merged, we can bring it into workhorse.