plugin: support local-based marketplace.json + install endpoint. (#13422)

Support marketplace.json that points to a local file, with
```
    "source":
    {
        "source": "local",
        "path": "./plugin-1"
    },
 ```
 
 Add a new plugin/install endpoint which add the plugin to the cache folder and enable it in config.toml.

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

@@ -264,6 +264,10 @@ client_request_definitions! {
         params: v2::SkillsConfigWriteParams,
         response: v2::SkillsConfigWriteResponse,
     },
+    PluginInstall => "plugin/install" {
+        params: v2::PluginInstallParams,
+        response: v2::PluginInstallResponse,
+    },
     TurnStart => "turn/start" {
         params: v2::TurnStartParams,
         inspect_params: true,