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

Cargo.toml

@@ -340,6 +340,7 @@ git2 = { version = "0.19", default-features = false }
 globset = "0.4"
 heed = { version = "0.20.1", features = ["read-txn-no-tls"] }
 hex = "0.4.3"
+hyper = "0.14"
 html5ever = "0.27.0"
 ignore = "0.4.22"
 image = "0.25.1"