feat: add customizable headers in proxy mode

[chanbengz]

Implement #2552. As suggested, I followed up the way that Proxy::basic_auth finishes its job by extending Proxy and ProxyScheme with a new field called misc. Spare me with the name misc because I can't come up with another word with four letter so that it aligns with host and auth.

I already tested with HTTP and HTTPS proxies but yet with Custom since I haven't taken a look at the custom part. So I will leave it to a future implementation.

Found that something broke with HeaderName::from_static and no one is using it, so I guess we're only able to use those pre-defined HTTP fields for now. It works fine but weird. Why does it have to be lower case?

[seanmonstar]

Thanks so much! I've been traveling (and so popping up for emergencies) but will do a proper review very soon.

[chanbengz]

Have a good trip!

[chanbengz]

I wrote a util macro to generate custom headers. Where should I put it? lib.rs for sure.

macro_rules! headers {
    () => (
        http::HeaderMap::new()
    );
    ($($key:expr => $value:expr),+ $(,)?) => ({
        let mut headers = http::HeaderMap::new();
        $(
            headers.insert($key, $value.parse().unwrap());
        )+
        headers
    });
}

So that we can do

let headers = headers![
    reqwest::header::CONTENT_TYPE => "application/json",
    reqwest::header::USER_AGENT => "reqwest",
];

[seanmonstar]

Nice work! Well done! Just one thing inline that I'd rather not include.

[chanbengz]

I didn't really get it but I dropped this macro. I guess I could be maintaining it for quite a long period. :)

[seanmonstar]

Hm, looks like CI is noticing a case where something is imported and not required?

[chanbengz]

Aha, it must be the auto-imported things by the editor during my development. I'll get that fixed soon!

And it's not the unused import problem. It's just that I'm missing the cfg(feature = ...) macro.

[chanbengz]

Done! Please click the CI button. It should be okay by now.

[seanmonstar]

Sorry I haven't merged yet, I've been working on hyperium/hyper#3850, to eventually refactor reqwest internals with it. I believe it should be fine (I expect the Proxy struct can include a hyper-util Matcher and optional extra headers as separate fields), but waiting a few weeks felt prudent.