Introduce a separate backend service for LLM calls (#15831)

This PR introduces a separate backend service for making LLM calls.

It exposes an HTTP interface that can be called by Zed clients. To call
these endpoints, the client must provide a `Bearer` token. These tokens
are issued/refreshed by the collab service over RPC.

We're adding this in a backwards-compatible way. Right now the access
tokens can only be minted for Zed staff, and calling this separate LLM
service is behind the `llm-service` feature flag (which is not
automatically enabled for Zed staff).

Release Notes:

- N/A

---------

Co-authored-by: Marshall <marshall@zed.dev>
Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

crates/collab/src/tests/test_server.rs

@@ -651,6 +651,7 @@ impl TestServer {
                 live_kit_server: None,
                 live_kit_key: None,
                 live_kit_secret: None,
+                llm_api_secret: None,
                 rust_log: None,
                 log_json: None,
                 zed_environment: "test".into(),