feat: implement CLI and SDK to support unregister shield resources in Llama Stack

Since in this [PR](https://github.com/llamastack/llama-stack/pull/2853) there was an extension of the Shields Protocol to allow the capability to unregister previously registered shields in Llama Stack, we need to create CLI and SDK for that. This is the original [issue](https://github.com/llamastack/llama-stack/issues/2581) in Llama Stack.

CC: @mattf 