serve nfs: new serve nfs command

Summary:
Adding a new command to serve any remote over NFS. This is only useful for new macOS versions where FUSE mounts are not available.
 * Added willscot/go-nfs dependency and updated go.mod and go.sum

Test Plan:
```
go run rclone.go serve nfs --http-url https://beta.rclone.org :http:
```

Test that it is serving correctly by mounting the NFS directory.

```
mkdir nfs-test
mount -oport=58654,mountport=58654 localhost: nfs-test
```

Then we can list the mounted directory to see it is working.
```
ls nfs-test
```

cmd/serve/nfs/nfs.go

@@ -0,0 +1,97 @@
+//go:build unix
+// +build unix
+
+// Package nfs implements a server to serve a VFS remote over NFSv3 protocol
+//
+// There is no authentication available on this server
+// and it is served on loopback interface by default.
+//
+// This is primarily used for mounting a VFS remote
+// in macOS, where FUSE-mounting mechanisms are usually not available.
+package nfs
+
+import (
+	"context"
+
+	"github.com/rclone/rclone/cmd"
+	"github.com/rclone/rclone/fs"
+	"github.com/rclone/rclone/fs/config/flags"
+	"github.com/rclone/rclone/fs/rc"
+	"github.com/rclone/rclone/vfs"
+	"github.com/rclone/rclone/vfs/vfsflags"
+	"github.com/spf13/cobra"
+	"github.com/spf13/pflag"
+)
+
+// Options contains options for the NFS Server
+type Options struct {
+	ListenAddr string // Port to listen on
+}
+
+var opt Options
+
+// AddFlags adds flags for the sftp
+func AddFlags(flagSet *pflag.FlagSet, Opt *Options) {
+	rc.AddOption("nfs", &Opt)
+	flags.StringVarP(flagSet, &Opt.ListenAddr, "addr", "", Opt.ListenAddr, "IPaddress:Port or :Port to bind server to", "")
+}
+
+func init() {
+	vfsflags.AddFlags(Command.Flags())
+	AddFlags(Command.Flags(), &opt)
+}
+
+// Run the command
+func Run(command *cobra.Command, args []string) {
+	var f fs.Fs
+	cmd.CheckArgs(1, 1, command, args)
+	f = cmd.NewFsSrc(args)
+	cmd.Run(false, true, command, func() error {
+		s, err := NewServer(context.Background(), vfs.New(f, &vfsflags.Opt), &opt)
+		if err != nil {
+			return err
+		}
+		return s.Serve()
+	})
+}
+
+// Command is the definition of the command
+var Command = &cobra.Command{
+	Use:   "nfs remote:path",
+	Short: `Serve the remote as an NFS mount`,
+	Long: `Create an NFS server that serves the given remote over the network.
+	
+The primary purpose for this command is to enable [mount command](/commands/rclone_mount/) on recent macOS versions where
+installing FUSE is very cumbersome. 
+
+Since this is running on NFSv3, no authentication method is available. Any client
+will be able to access the data. To limit access, you can use serve NFS on loopback address
+and rely on secure tunnels (such as SSH). For this reason, by default, a random TCP port is chosen and loopback interface is used for the listening address;
+meaning that it is only available to the local machine. If you want other machines to access the
+NFS mount over local network, you need to specify the listening address and port using ` + "`--addr`" + ` flag.
+
+Modifying files through NFS protocol requires VFS caching. Usually you will need to specify ` + "`--vfs-cache-mode`" + `
+in order to be able to write to the mountpoint (full is recommended). If you don't specify VFS cache mode,
+the mount will be read-only.
+
+To serve NFS over the network use following command:
+
+    rclone serve nfs remote: --addr 0.0.0.0:$PORT --vfs-cache-mode=full
+
+We specify a specific port that we can use in the mount command:
+
+To mount the server under Linux/macOS, use the following command:
+    
+    mount -oport=$PORT,mountport=$PORT $HOSTNAME: path/to/mountpoint
+
+Where ` + "`$PORT`" + ` is the same port number we used in the serve nfs command.
+
+This feature is only available on Unix platforms.
+
+` + vfs.Help,
+	Annotations: map[string]string{
+		"versionIntroduced": "v1.65",
+		"groups":            "Filter",
+	},
+	Run: Run,
+}