Implements proxy_headers method to add headers only to the proxy.

[jcamiel]

This PR re-enables the proxy_headers method (see https://curl.se/libcurl/c/CURLOPT_PROXYHEADER.html available since curl 7.37.0)

It is basically the same code as http_headers.

The rational for this PR is to be used in Hurl, a cli HTTP engine powered by curl-rust. Hurl exposes various curl commands, basically delegating everything to curl-rust/libcurl and adding some syntax to test HTTP responses. One issue Orange-OpenSource/hurl#3944 and one draft PR Orange-OpenSource/hurl#4482 needs to use this new method exposed by curl-rust.

To test it, I've used this sample:

use curl::easy::{Easy, List};

#[test]
fn test_proxy_header() {
    let mut easy = Easy::new();
    easy.url("https://google.com").unwrap();
    easy.ssl_verify_peer(false).unwrap();

    let mut headers = List::new();
    headers.append("Proxy-Only: foo").unwrap();
    easy.proxy_headers(headers).unwrap();

    let mut headers = List::new();
    headers.append("All: bar").unwrap();
    easy.http_headers(headers).unwrap();

    easy.proxy("http://127.0.0.1:8080").unwrap();
    easy.verbose(true).unwrap();
    easy.write_function(|data| Ok(data.len())).unwrap();
    easy.perform().unwrap();
}

And with mitmproxy:

$ mitmproxy

We can see that only "All: bar" header is sent to the target :