Optional Transformer: Elevate resource hints to header

## Feature description

If we prepare a response in WP and the header has not yet been sent, we could run an additional transformer to elevate the resource hints (`` elements in head) to the header as `Link: ...` entries: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Link

This would have these resource hints be active even when a URL from our origin is only sent as header for a prefetch.

This is done by the Amp package for signed exchanges: https://github.com/ampproject/amppackager/blob/cc38c5fad40fc603119a298700820b97a4f0c54f/transformer/transformer.go#L194

I'm unsure whether that would be the case and whether we'd actually profit from this.

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## Acceptance criteria

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## Implementation brief

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## QA testing instructions

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## Demo

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## Changelog entry

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