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/handler.go

@@ -0,0 +1,70 @@
+//go:build unix
+// +build unix
+
+package nfs
+
+import (
+	"context"
+	"net"
+
+	"github.com/go-git/go-billy/v5"
+	"github.com/rclone/rclone/vfs"
+	"github.com/willscott/go-nfs"
+	nfshelper "github.com/willscott/go-nfs/helpers"
+)
+
+// NewBackendAuthHandler creates a handler for the provided filesystem
+func NewBackendAuthHandler(vfs *vfs.VFS) nfs.Handler {
+	return &BackendAuthHandler{vfs}
+}
+
+// BackendAuthHandler returns a NFS backing that exposes a given file system in response to all mount requests.
+type BackendAuthHandler struct {
+	vfs *vfs.VFS
+}
+
+// Mount backs Mount RPC Requests, allowing for access control policies.
+func (h *BackendAuthHandler) Mount(ctx context.Context, conn net.Conn, req nfs.MountRequest) (status nfs.MountStatus, hndl billy.Filesystem, auths []nfs.AuthFlavor) {
+	status = nfs.MountStatusOk
+	hndl = &FS{vfs: h.vfs}
+	auths = []nfs.AuthFlavor{nfs.AuthFlavorNull}
+	return
+}
+
+// Change provides an interface for updating file attributes.
+func (h *BackendAuthHandler) Change(fs billy.Filesystem) billy.Change {
+	if c, ok := fs.(billy.Change); ok {
+		return c
+	}
+	return nil
+}
+
+// FSStat provides information about a filesystem.
+func (h *BackendAuthHandler) FSStat(ctx context.Context, f billy.Filesystem, s *nfs.FSStat) error {
+	total, _, free := h.vfs.Statfs()
+	s.TotalSize = uint64(total)
+	s.FreeSize = uint64(free)
+	s.AvailableSize = uint64(free)
+	return nil
+}
+
+// ToHandle handled by CachingHandler
+func (h *BackendAuthHandler) ToHandle(f billy.Filesystem, s []string) []byte {
+	return []byte{}
+}
+
+// FromHandle handled by CachingHandler
+func (h *BackendAuthHandler) FromHandle([]byte) (billy.Filesystem, []string, error) {
+	return nil, []string{}, nil
+}
+
+// HandleLimit handled by cachingHandler
+func (h *BackendAuthHandler) HandleLimit() int {
+	return -1
+}
+
+func newHandler(vfs *vfs.VFS) nfs.Handler {
+	handler := NewBackendAuthHandler(vfs)
+	cacheHelper := nfshelper.NewCachingHandler(handler, 1024)
+	return cacheHelper
+}