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/v2.rs

@@ -2546,6 +2546,21 @@ pub struct SkillsConfigWriteResponse {
     pub effective_enabled: bool,
 }
 
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct PluginInstallParams {
+    pub marketplace_name: String,
+    pub plugin_name: String,
+    #[ts(optional = nullable)]
+    pub cwd: Option<PathBuf>,
+}
+
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct PluginInstallResponse {}
+
 impl From<CoreSkillMetadata> for SkillMetadata {
     fn from(value: CoreSkillMetadata) -> Self {
         Self {