collab: Attach GeoIP country code to RPC sessions (#15814)

This PR updates collab to attach the user's GeoIP country code to their
RPC session.

We source the country code from the
[`CF-IPCountry`](https://developers.cloudflare.com/fundamentals/reference/http-request-headers/#cf-ipcountry)
header.

Release Notes:

- N/A

crates/collab/src/api.rs

@@ -14,7 +14,8 @@ use anyhow::anyhow;
 use axum::{
     body::Body,
     extract::{Path, Query},
-    http::{self, Request, StatusCode},
+    headers::Header,
+    http::{self, HeaderName, Request, StatusCode},
     middleware::{self, Next},
     response::IntoResponse,
     routing::{get, post},
@@ -22,11 +23,44 @@ use axum::{
 };
 use axum_extra::response::ErasedJson;
 use serde::{Deserialize, Serialize};
-use std::sync::Arc;
+use std::sync::{Arc, OnceLock};
 use tower::ServiceBuilder;
 
 pub use extensions::fetch_extensions_from_blob_store_periodically;
 
+pub struct CloudflareIpCountryHeader(String);
+
+impl Header for CloudflareIpCountryHeader {
+    fn name() -> &'static HeaderName {
+        static CLOUDFLARE_IP_COUNTRY_HEADER: OnceLock<HeaderName> = OnceLock::new();
+        CLOUDFLARE_IP_COUNTRY_HEADER.get_or_init(|| HeaderName::from_static("cf-ipcountry"))
+    }
+
+    fn decode<'i, I>(values: &mut I) -> Result<Self, axum::headers::Error>
+    where
+        Self: Sized,
+        I: Iterator<Item = &'i axum::http::HeaderValue>,
+    {
+        let country_code = values
+            .next()
+            .ok_or_else(axum::headers::Error::invalid)?
+            .to_str()
+            .map_err(|_| axum::headers::Error::invalid())?;
+
+        Ok(Self(country_code.to_string()))
+    }
+
+    fn encode<E: Extend<axum::http::HeaderValue>>(&self, _values: &mut E) {
+        unimplemented!()
+    }
+}
+
+impl std::fmt::Display for CloudflareIpCountryHeader {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "{}", self.0)
+    }
+}
+
 pub fn routes(rpc_server: Option<Arc<rpc::Server>>, state: Arc<AppState>) -> Router<(), Body> {
     Router::new()
         .route("/user", get(get_authenticated_user))