Authorize access to language model providers based on country (#15859)

This PR updates the LLM service to authorize access to language model
providers based on the requester's country.

We detect the country using Cloudflare's
[`CF-IPCountry`](https://developers.cloudflare.com/fundamentals/reference/http-request-headers/#cf-ipcountry)
header.

The country code is then checked against the list of supported countries
for the given LLM provider. Countries that are not supported will
receive an `HTTP 451: Unavailable For Legal Reasons` response.

Release Notes:

- N/A

crates/open_ai/src/open_ai.rs

@@ -1,3 +1,5 @@
+mod supported_countries;
+
 use anyhow::{anyhow, Context, Result};
 use futures::{io::BufReader, stream::BoxStream, AsyncBufReadExt, AsyncReadExt, Stream, StreamExt};
 use http_client::{AsyncBody, HttpClient, Method, Request as HttpRequest};
@@ -7,6 +9,8 @@ use serde_json::Value;
 use std::{convert::TryFrom, future::Future, time::Duration};
 use strum::EnumIter;
 
+pub use supported_countries::*;
+
 pub const OPEN_AI_API_URL: &str = "https://api.openai.com/v1";
 
 fn is_none_or_empty<T: AsRef<[U]>, U>(opt: &Option<T>) -> bool {