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/ClientRequest.json

@@ -951,6 +951,27 @@
       ],
       "type": "string"
     },
+    "PluginInstallParams": {
+      "properties": {
+        "cwd": {
+          "type": [
+            "string",
+            "null"
+          ]
+        },
+        "marketplaceName": {
+          "type": "string"
+        },
+        "pluginName": {
+          "type": "string"
+        }
+      },
+      "required": [
+        "marketplaceName",
+        "pluginName"
+      ],
+      "type": "object"
+    },
     "ProductSurface": {
       "enum": [
         "chatgpt",
@@ -3339,6 +3360,30 @@
       "title": "Skills/config/writeRequest",
       "type": "object"
     },
+    {
+      "properties": {
+        "id": {
+          "$ref": "#/definitions/RequestId"
+        },
+        "method": {
+          "enum": [
+            "plugin/install"
+          ],
+          "title": "Plugin/installRequestMethod",
+          "type": "string"
+        },
+        "params": {
+          "$ref": "#/definitions/PluginInstallParams"
+        }
+      },
+      "required": [
+        "id",
+        "method",
+        "params"
+      ],
+      "title": "Plugin/installRequest",
+      "type": "object"
+    },
     {
       "properties": {
         "id": {