[POC, DNM] Expose socks proxy (per user-v2 net, not per instance)

Usage:
```
curl \
  --proxy socks5h://localhost/$HOME/.lima/_networks/user-v2/user-v2_socks.sock \
  192.168.104.4
```

This is similar to the `limactl tunnel` proposal (PR 2710).
While PR 2710 creates a proxy per an instance, this commit creates a
proxy per a user-v2 network.

Remarks:
- Only works for user-v2 networks.
- DNS lookup is not implemented yet in this POC.
  Could be taken from https://github.com/norouter/norouter/blob/v0.6.5/pkg/agent/socks/socks.go#L57-L75
- https://github.com/cybozu-go/usocksd is a dependency hog (See the `go.mod` diff).
  Should be replaced with a fork or another library.

Overall, PR 2710 might be better than this commit, as PR 2710 works for
any network driver, does not need an additional DNS resolver, and
does not incur additional `go.mod` deps.

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

cmd/limactl/usernet.go

@@ -22,6 +22,7 @@ func newUsernetCommand() *cobra.Command {
 	hostagentCommand.Flags().StringP("endpoint", "e", "", "exposes usernet api(s) on this endpoint")
 	hostagentCommand.Flags().String("listen-qemu", "", "listen for qemu connections")
 	hostagentCommand.Flags().String("listen", "", "listen on a Unix socket and receive Bess-compatible FDs as SCM_RIGHTS messages")
+	hostagentCommand.Flags().String("listen-socks", "", "listen for socks connectioss")
 	hostagentCommand.Flags().String("subnet", "192.168.5.0/24", "sets subnet value for the usernet network")
 	hostagentCommand.Flags().Int("mtu", 1500, "mtu")
 	hostagentCommand.Flags().StringToString("leases", nil, "pass default static leases for startup. Eg: '192.168.104.1=52:55:55:b3:bc:d9,192.168.104.2=5a:94:ef:e4:0c:df' ")
@@ -54,6 +55,10 @@ func usernetAction(cmd *cobra.Command, _ []string) error {
 	if err != nil {
 		return err
 	}
+	socksSocket, err := cmd.Flags().GetString("listen-socks")
+	if err != nil {
+		return err
+	}
 	subnet, err := cmd.Flags().GetString("subnet")
 	if err != nil {
 		return err
@@ -72,6 +77,7 @@ func usernetAction(cmd *cobra.Command, _ []string) error {
 	os.RemoveAll(endpoint)
 	os.RemoveAll(qemuSocket)
 	os.RemoveAll(fdSocket)
+	os.RemoveAll(socksSocket)
 
 	// Environment Variables
 	// LIMA_USERNET_RESOLVE_IP_ADDRESS_TIMEOUT: Specifies the timeout duration for resolving IP addresses in minutes. Default is 2 minutes.
@@ -81,6 +87,7 @@ func usernetAction(cmd *cobra.Command, _ []string) error {
 		Endpoint:      endpoint,
 		QemuSocket:    qemuSocket,
 		FdSocket:      fdSocket,
+		SocksSocket:   socksSocket,
 		Subnet:        subnet,
 		DefaultLeases: leases,
 	})

go.mod

@@ -55,6 +55,31 @@ require (
 	k8s.io/client-go v0.31.1
 )
 
+require (
+	github.com/beorn7/perks v1.0.1 // indirect
+	github.com/cespare/xxhash/v2 v2.3.0 // indirect
+	github.com/cybozu-go/log v1.6.1 // indirect
+	github.com/cybozu-go/netutil v1.4.2 // indirect
+	github.com/cybozu-go/usocksd v1.3.0 // indirect
+	github.com/cybozu-go/well v1.11.0 // indirect
+	github.com/hashicorp/hcl v1.0.0 // indirect
+	github.com/matttproud/golang_protobuf_extensions v1.0.4 // indirect
+	github.com/onsi/gomega v1.34.1 // indirect
+	github.com/pelletier/go-toml v1.9.5 // indirect
+	github.com/prometheus/client_golang v1.14.0 // indirect
+	github.com/prometheus/client_model v0.3.0 // indirect
+	github.com/prometheus/common v0.37.0 // indirect
+	github.com/prometheus/procfs v0.8.0 // indirect
+	github.com/spf13/afero v1.8.2 // indirect
+	github.com/spf13/cast v1.5.0 // indirect
+	github.com/spf13/jwalterweatherman v1.1.0 // indirect
+	github.com/spf13/viper v1.12.0 // indirect
+	github.com/subosito/gotenv v1.4.0 // indirect
+	github.com/vishvananda/netlink v1.3.0 // indirect
+	github.com/vishvananda/netns v0.0.4 // indirect
+	gopkg.in/ini.v1 v1.66.6 // indirect
+)
+
 require (
 	github.com/Code-Hex/go-infinity-channel v1.0.0 // indirect
 	github.com/VividCortex/ewma v1.2.0 // indirect