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/tests/test_server.rs

@@ -244,6 +244,7 @@ impl TestServer {
                                 client_name,
                                 Principal::User(user),
                                 ZedVersion(SemanticVersion::new(1, 0, 0)),
+                                None,
                                 Some(connection_id_tx),
                                 Executor::Deterministic(cx.background_executor().clone()),
                             ))
@@ -377,6 +378,7 @@ impl TestServer {
                                 "dev-server".to_string(),
                                 Principal::DevServer(dev_server),
                                 ZedVersion(SemanticVersion::new(1, 0, 0)),
+                                None,
                                 Some(connection_id_tx),
                                 Executor::Deterministic(cx.background_executor().clone()),
                             ))