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/google_ai/src/google_ai.rs

@@ -1,8 +1,12 @@
+mod supported_countries;
+
 use anyhow::{anyhow, Result};
 use futures::{io::BufReader, stream::BoxStream, AsyncBufReadExt, AsyncReadExt, Stream, StreamExt};
 use http_client::HttpClient;
 use serde::{Deserialize, Serialize};
 
+pub use supported_countries::*;
+
 pub const API_URL: &str = "https://generativelanguage.googleapis.com";
 
 pub async fn stream_generate_content(