Added dns_overrides_callback

[vitalka200]

Hi All!

I'd like to extend the DNS resolve functionality added by #1277.
It will allow dynamically updating the DNS overrides and possibly be an alternative workaround for #1421.

Having dns_overrides callback exposed, one can implement something like in the example below.

use std::net::SocketAddr;
use std::collections::HashMap;
use std::sync::{Arc, Mutex};
use reqwest::Client;

struct ClientWrapper {
    client: Client,
    dns_overrides: Arc<Mutex<HashMap<String, SocketAddr>>>,
}

impl ClientWrapper {

    pub fn new() -> ClientWrapper {
        let dns_overrides: Arc<Mutex<HashMap<String, SocketAddr>>> = Arc::new(Mutex::new(HashMap::new()));
        let overrides = dns_overrides.clone();
        ClientWrapper {
            client: Client::builder().resolve_callback(move |name| {
                let overrides = overrides.lock().unwrap();
                (*overrides).get(name).cloned()
            }).build().unwrap(),
            dns_overrides
        }
    }

    pub fn new_with_predefined(overrides: HashMap<String, SocketAddr>) -> ClientWrapper {
        let dns_overrides: Arc<Mutex<HashMap<String, SocketAddr>>> = Arc::new(Mutex::new(overrides));
        let overrides = dns_overrides.clone();
        ClientWrapper {
            client: Client::builder().resolve_callback(move |name| {
                let overrides = overrides.lock().unwrap();
                (*overrides).get(name).cloned()
            }).build().unwrap(),
            dns_overrides
        }
    }

    pub fn add_resolve(&self, name: &str, addr: SocketAddr) {
        let dns_overrides = &mut self.dns_overrides.lock().unwrap();
        (*dns_overrides).insert(name.to_string(), addr);
    }

    pub fn remove_resolve(&self, name: &str) {
        let dns_overrides = &mut self.dns_overrides.lock().unwrap();
        (*dns_overrides).remove(name);
    }
}

// Predefined resolves
let wrapper = ClientWrapper::new_with_predefined([
        ("example.com".to_string(), "127.0.0.10".parse::<SocketAddr>().unwrap())
    ].iter().cloned().collect()
);

// Let's suppose that it will fail as the address is wrong
let failed_response = wrapper.client
    .get("http://example.com")
    .send()
    .await;

// So let's fix the address by replacing resolve with the correct IP
wrapper.add_resolve("example.com", "127.0.0.1".parse().unwrap());
let succeded_response = wrapper.client
.get("http://example.com")
.send()
.await;
    
// Let's remove resolve to not explode the memory
wrapper.remove_resolve("example.com");

[tasn]

Duplicate (though maybe an improvement) of #1441?

@seanmonstar, can you please take a look? We would love to have this too. 🙏🏻

[vitalka200]

Implemented in #1653