support plugin/list. (#13540)

Introduce a plugin/list which reads from local marketplace.json.
Also update the signature for plugin/install.

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

@@ -248,6 +248,10 @@ client_request_definitions! {
         params: v2::SkillsListParams,
         response: v2::SkillsListResponse,
     },
+    PluginList => "plugin/list" {
+        params: v2::PluginListParams,
+        response: v2::PluginListResponse,
+    },
     SkillsRemoteList => "skills/remote/list" {
         params: v2::SkillsRemoteReadParams,
         response: v2::SkillsRemoteReadResponse,