Refactor plugin config and cache path (#13333)

Update config.toml plugin entries to use
<plugin_name>@<marketplace_name> as the key.
Plugin now stays in
[plugins/cache/marketplace-name/plugin-name/$version/]
Clean up the plugin code structure.
Add plugin install functionality (not used yet).

codex-rs/core/config.schema.json

@@ -1107,14 +1107,8 @@
         "enabled": {
           "default": true,
           "type": "boolean"
-        },
-        "path": {
-          "$ref": "#/definitions/AbsolutePathBuf"
         }
       },
-      "required": [
-        "path"
-      ],
       "type": "object"
     },
     "ProjectConfig": {