fetch: send 'sec-purpose' header for prefetch requests

[trivikr]

This relates to...

Step 14 in WHATWG fetch specification https://fetch.spec.whatwg.org/#http-network-or-cache-fetch, added in https://github.com/whatwg/fetch/pull/1576

Changes

Sends 'sec-purpose' header for prefetch requests as per WHATWG fetch specification

Features

N/A

Bug Fixes

N/A

Breaking Changes and Deprecations

N/A

Status

• [x] I have read and agreed to the Developer's Certificate of Origin
• [ ] Tested
• [ ] Benchmarked (optional)
• [ ] Documented
• [ ] Review ready
• [ ] In review
• [ ] Merge ready

[KhafraDev]

There are no viable WPTs for server implementations

[Uzlopak]

@trivikr I pushed changes, PTAL