feat: Deno.createHttpClient defaultHeaders

[crowlKats]

This adds an option to specify headers in createHttpClient, without any filters.
This is useful for setting forbidden headers (ie host header).
Closes #16840
Ref #11017

[bartlomieju]

LGTM, since it's an unstable API and this seems very useful.

[andreubotella]

Wouldn't it be better to have an allowHost option to allow specifying a Host header in the request without the filtering? I know this doesn't currently work because the Deno.HttpClient is only passed on the call to fetch, rather than on request construction, but it seems like client could be part of RequestInit just fine. Note that response headers aren't filtered in Deno.

Note that this issue might be relevant to the work we're doing in WinterCG with forking the Fetch spec to cover the needs of server-side runtimes.

[bartlomieju]

Good point @andreubotella, I'm gonna defer to @lucacasonato to decide.

[lucacasonato]

I agree with the allowHost option. It should be passed on the Deno.HttpClient. Request should not filter out the Host header in JS, but rather it should be filtered in Rust based on whether the client has this option set or not.

Because of this it should not matter that Request does not have a client option, because it would never affect the JS side of Request, only ever the fetch itself.

FYI on the footgun that caused us to block this in the first place: denoland/deploy_feedback#52

[crowlKats]

Closing in favour of #19689