Consider adding a new header for server-side detection of speculation-rules initiated requests

[SulemanAhmadd]

Currently, both prefetch requests initiated through Resource Hints (like the rel=prefetch attribute) and those triggered by speculation rules include the Sec-Purpose: prefetch request header. While there’s no significant semantic difference between these requests (as both are for prefetching resource), servers, especially in CDN use cases where speculation rules may be deployed on behalf of customers alongside customer-specific link prefetching, need a way to distinguish between the two.

It would be useful for servers to have a mechanism to differentiate between prefetch requests initiated by speculation rules vs initiated by other methods. I believe tag values defined in #336 can be helpful for this use-case as well.