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_unsupported.go

@@ -0,0 +1,13 @@
+// For unsupported architectures
+//go:build !unix
+// +build !unix
+
+// Package nfs is not supported on non-Unix platforms
+package nfs
+
+import (
+	"github.com/spf13/cobra"
+)
+
+// For unsupported platforms we just put nil
+var Command *cobra.Command = nil