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/billing.rs

@@ -120,7 +120,7 @@ async fn create_billing_subscription(
         .zip(app.config.stripe_price_id.clone())
     else {
         log::error!("failed to retrieve Stripe client or price ID");
-        Err(Error::Http(
+        Err(Error::http(
             StatusCode::NOT_IMPLEMENTED,
             "not supported".into(),
         ))?
@@ -201,7 +201,7 @@ async fn manage_billing_subscription(
 
     let Some(stripe_client) = app.stripe_client.clone() else {
         log::error!("failed to retrieve Stripe client");
-        Err(Error::Http(
+        Err(Error::http(
             StatusCode::NOT_IMPLEMENTED,
             "not supported".into(),
         ))?