Develop Slirp backend for Flannel

* slirp4netns allows connecting a network namespace to host in a completely unprivileged way.
Practically, this handles container-to-host communication.

* Flannel is a simple and easy way to configure a layer 3 network fabric designed for Kubernetes.
Practically this handles host-to-host communication.

So what we need, it to develop a slirp backend for Flannel.

https://github.com/mgoltzsche/slirp-cni-plugin/blob/master/slirp.go

https://github.com/flannel-io/flannel/blob/629cd70d816e56853aac967f92ed3dade7275baf/pkg/backend/hostgw/hostgw.go

https://github.com/containernetworking/plugins/blob/main/plugins/main/tap/tap.go