Rancher Extensions have arbitrary file access via path traversal

A vulnerability has been identified in [Rancher's Extensions](https://ranchermanager.docs.rancher.com/integrations-in-rancher/rancher-extensions) where malicious code can be injected in Rancher through a path traversal in the `compressedEndpoint` field inside a `UIPlugin` deployment. A malicious UI extension could abuse that to: * Overwrite Rancher binaries or configuration to inject code. * Write to /var/lib/rancher/ to tamper with cluster state. * If hostPath volumes are mounted, write to the host node filesystem. * Use this issue to chain with other attack vectors.