Podman: support Pasta mode

[AkihiroSuda]

Usernetes currently doesn't seem to work with Pasta mode

usernetes/README.md

Lines 84 to 101 in d118655

	- slirp4netns, not Pasta:
	```
	# Podman v5 (or later) users have to change the network mode from pasta to slirp4netns.
	# This step is not needed for Docker, nerdctl, and Podman v4.
	mkdir -p "$HOME/.config/containers/containers.conf.d"
	cat <<EOF >"$HOME/.config/containers/containers.conf.d/slirp4netns.conf"
	[network]
	default_rootless_network_cmd="slirp4netns"
	EOF
	```
	<!--
	pasta does not seem to work well
	> 2024-12-02T17:15:40.070018488Z stderr F E1202 17:15:40.068621 1 main.go:228] Failed to create SubnetManager:
	> error retrieving pod spec for 'kube-flannel/kube-flannel-ds-ms2d9': Get "https://10.96.0.1:443/api/v1/namespaces/kube-flannel/pods/kube-flannel-ds-ms2d9":
	> dial tcp 10.96.0.1:443: i/o timeout
	-->

Upstream issue:

• Usernetes doesn't work with Pasta mode of Podman (works fine with slirp4netns mode): Get "https://10.96.0.1:443/api/v1/namespaces/kube-flannel/pods/kube-flannel-ds-pnnrt": dial tcp 10.96.0.1:443: i/o timeout containers/container-libs#55

[AkihiroSuda]

Looks like we have to fix this before the release of Podman v6

• Usernetes doesn't work with Pasta mode of Podman (works fine with slirp4netns mode): Get "https://10.96.0.1:443/api/v1/namespaces/kube-flannel/pods/kube-flannel-ds-pnnrt": dial tcp 10.96.0.1:443: i/o timeout containers/container-libs#55 (comment)