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/proto/proto/zed.proto

@@ -126,7 +126,7 @@ message Envelope {
         Unfollow unfollow = 101;
         GetPrivateUserInfo get_private_user_info = 102;
         GetPrivateUserInfoResponse get_private_user_info_response = 103;
-        UpdateUserPlan update_user_plan = 234; // current max
+        UpdateUserPlan update_user_plan = 234;
         UpdateDiffBase update_diff_base = 104;
 
         OnTypeFormatting on_type_formatting = 105;
@@ -270,6 +270,9 @@ message Envelope {
 
         AddWorktree add_worktree = 222;
         AddWorktreeResponse add_worktree_response = 223;
+
+        GetLlmToken get_llm_token = 235;
+        GetLlmTokenResponse get_llm_token_response = 236; // current max
     }
 
     reserved 158 to 161;
@@ -2425,6 +2428,12 @@ message SynchronizeContextsResponse {
     repeated ContextVersion contexts = 1;
 }
 
+message GetLlmToken {}
+
+message GetLlmTokenResponse {
+    string token = 1;
+}
+
 // Remote FS
 
 message AddWorktree {

crates/proto/src/proto.rs

@@ -259,6 +259,8 @@ messages!(
     (GetTypeDefinitionResponse, Background),
     (GetImplementation, Background),
     (GetImplementationResponse, Background),
+    (GetLlmToken, Background),
+    (GetLlmTokenResponse, Background),
     (GetUsers, Foreground),
     (Hello, Foreground),
     (IncomingCall, Foreground),
@@ -438,6 +440,7 @@ request_messages!(
     (GetImplementation, GetImplementationResponse),
     (GetDocumentHighlights, GetDocumentHighlightsResponse),
     (GetHover, GetHoverResponse),
+    (GetLlmToken, GetLlmTokenResponse),
     (GetNotifications, GetNotificationsResponse),
     (GetPrivateUserInfo, GetPrivateUserInfoResponse),
     (GetProjectSymbols, GetProjectSymbolsResponse),