Stop leaking isahc assumption (#18408)

Users of our http_client crate knew they were interacting with isahc as
they set its extensions on the request. This change adds our own
equivalents for their APIs in preparation for changing the default http
client.

Release Notes:

- N/A

crates/copilot/src/copilot_chat.rs

@@ -7,8 +7,7 @@ use chrono::DateTime;
 use fs::Fs;
 use futures::{io::BufReader, stream::BoxStream, AsyncBufReadExt, AsyncReadExt, StreamExt};
 use gpui::{AppContext, AsyncAppContext, Global};
-use http_client::{AsyncBody, HttpClient, Method, Request as HttpRequest};
-use isahc::config::Configurable;
+use http_client::{AsyncBody, HttpClient, HttpRequestExt, Method, Request as HttpRequest};
 use paths::home_dir;
 use serde::{Deserialize, Serialize};
 use settings::watch_config_file;
@@ -275,7 +274,7 @@ async fn request_api_token(
         .header("Accept", "application/json");
 
     if let Some(low_speed_timeout) = low_speed_timeout {
-        request_builder = request_builder.low_speed_timeout(100, low_speed_timeout);
+        request_builder = request_builder.read_timeout(low_speed_timeout);
     }
 
     let request = request_builder.body(AsyncBody::empty())?;
@@ -332,7 +331,7 @@ async fn stream_completion(
         .header("Copilot-Integration-Id", "vscode-chat");
 
     if let Some(low_speed_timeout) = low_speed_timeout {
-        request_builder = request_builder.low_speed_timeout(100, low_speed_timeout);
+        request_builder = request_builder.read_timeout(low_speed_timeout);
     }
     let request = request_builder.body(AsyncBody::from(serde_json::to_string(&request)?))?;
     let mut response = client.send(request).await?;