Add support for describeInstance

Hi all,

Firecracker-containerd `GetVMInfo` API doesn't really call the VM API, we want to periodically call some VM API to check if instance is still responsive, we'd like to use [describeInstance](https://github.com/firecracker-microvm/firecracker/blob/29f81f1e72361329aa09c9db0d87e68d3d091fcf/api_server/swagger/firecracker.yaml#L29) API for this purpose, but it doesn't seem it's implemented, do you have plan to implement it? Apologize ahead if you already done this.

Thanks,
Haikuo