Why this change?
In https://web.dev/articles/preconnect-and-dns-prefetch, it describes
how hinting to the browser to preconnect to domains which we will
eventually use the connection for can help improve the time it takes to
load a page.
We are putting this behind an experimental flag so that we can test and
profile this in a production environment.
What does this change introduce?
Introduce a hidden experimental `experimental_preconnect_link_header`
site setting which when enabled will add the `preconnect` and
`dns-prefetch` resource hints to the response headers for full page load
requests.
36cdb1444c