[connectors] Support connectors part 1 - App server & MCP (#9667)

In order to make Codex work with connectors, we add a built-in gateway
MCP that acts as a transparent proxy between the client and the
connectors. The gateway MCP collects actions that are accessible to the
user and sends them down to the user, when a connector action is chosen
to be called, the client invokes the action through the gateway MCP as
well.

 - [x] Add the system built-in gateway MCP to list and run connectors.
 - [x] Add the app server methods and protocol

codex-rs/app-server-protocol/src/protocol/common.rs

@@ -137,6 +137,10 @@ client_request_definitions! {
         params: v2::SkillsListParams,
         response: v2::SkillsListResponse,
     },
+    AppsList => "app/list" {
+        params: v2::AppsListParams,
+        response: v2::AppsListResponse,
+    },
     SkillsConfigWrite => "skills/config/write" {
         params: v2::SkillsConfigWriteParams,
         response: v2::SkillsConfigWriteResponse,