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/schema/json/v2/PluginInstallResponse.json

@@ -0,0 +1,5 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "title": "PluginInstallResponse",
+  "type": "object"
+}