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/api/extensions.rs

@@ -185,7 +185,7 @@ async fn download_extension(
         .clone()
         .zip(app.config.blob_store_bucket.clone())
     else {
-        Err(Error::Http(
+        Err(Error::http(
             StatusCode::NOT_IMPLEMENTED,
             "not supported".into(),
         ))?
@@ -202,7 +202,7 @@ async fn download_extension(
         .await?;
 
     if !version_exists {
-        Err(Error::Http(
+        Err(Error::http(
             StatusCode::NOT_FOUND,
             "unknown extension version".into(),
         ))?;