Add support for HTTP proxy in network commands (#10401)

Closes #8847

# Description

If the `HTTP_PROXY` variable is found, use its value to setup ureq
proxy. I haven't implemented `NO_PROXY` at the moment.

# User-Facing Changes

No breaking change for the user, the network commands simply use an
environment variable.

# Tests + Formatting

The existing tests seem to run fine, although I can't think of a new
test to add.

crates/nu-command/src/network/http/client.rs

@@ -34,10 +34,17 @@ pub fn http_client(allow_insecure: bool) -> ureq::Agent {
         .build()
         .expect("Failed to build network tls");
 
-    ureq::builder()
+    let mut agent_builder = ureq::builder()
         .user_agent("nushell")
-        .tls_connector(std::sync::Arc::new(tls))
-        .build()
+        .tls_connector(std::sync::Arc::new(tls));
+
+    if let Some(http_proxy) = retrieve_http_proxy_from_env() {
+        if let Ok(proxy) = ureq::Proxy::new(http_proxy) {
+            agent_builder = agent_builder.proxy(proxy);
+        }
+    };
+
+    agent_builder.build()
 }
 
 pub fn http_parse_url(
@@ -639,3 +646,11 @@ pub fn request_handle_response_headers(
         },
     }
 }
+
+fn retrieve_http_proxy_from_env() -> Option<String> {
+    std::env::vars()
+        .find(|(key, _)| key == "http_proxy")
+        .or(std::env::vars().find(|(key, _)| key == "HTTP_PROXY"))
+        .or(std::env::vars().find(|(key, _)| key == "ALL_PROXY"))
+        .map(|(_, value)| value)
+}