sftp: add --sftp-ssh to specify an external ssh binary to use

This allows using an external ssh binary instead of the built in ssh
library for making SFTP connections.

This makes another integration test target TestSFTPRcloneSSH:

Fixes #7012

backend/sftp/ssh_internal.go

@@ -0,0 +1,90 @@
+//go:build !plan9
+// +build !plan9
+
+package sftp
+
+import (
+	"context"
+	"io"
+
+	"github.com/rclone/rclone/fs"
+	"github.com/rclone/rclone/fs/fshttp"
+	"golang.org/x/crypto/ssh"
+)
+
+// Internal ssh connections with "golang.org/x/crypto/ssh"
+
+type sshClientInternal struct {
+	srv *ssh.Client
+}
+
+// newSSHClientInternal starts a client connection to the given SSH server. It is a
+// convenience function that connects to the given network address,
+// initiates the SSH handshake, and then sets up a Client.
+func (f *Fs) newSSHClientInternal(ctx context.Context, network, addr string, sshConfig *ssh.ClientConfig) (sshClient, error) {
+	dialer := fshttp.NewDialer(ctx)
+	conn, err := dialer.Dial(network, addr)
+	if err != nil {
+		return nil, err
+	}
+	c, chans, reqs, err := ssh.NewClientConn(conn, addr, sshConfig)
+	if err != nil {
+		return nil, err
+	}
+	fs.Debugf(f, "New connection %s->%s to %q", c.LocalAddr(), c.RemoteAddr(), c.ServerVersion())
+	srv := ssh.NewClient(c, chans, reqs)
+	return sshClientInternal{srv}, nil
+}
+
+// Wait for connection to close
+func (s sshClientInternal) Wait() error {
+	return s.srv.Conn.Wait()
+}
+
+// Send a keepalive over the ssh connection
+func (s sshClientInternal) SendKeepAlive() {
+	_, _, err := s.srv.SendRequest("keepalive@openssh.com", true, nil)
+	if err != nil {
+		fs.Debugf(nil, "Failed to send keep alive: %v", err)
+	}
+}
+
+// Close the connection
+func (s sshClientInternal) Close() error {
+	return s.srv.Close()
+}
+
+// CanReuse indicates if this client can be reused
+func (s sshClientInternal) CanReuse() bool {
+	return true
+}
+
+// Check interfaces
+var _ sshClient = sshClientInternal{}
+
+// Thin wrapper for *ssh.Session to implement sshSession interface
+type sshSessionInternal struct {
+	*ssh.Session
+}
+
+// Set the stdout
+func (s sshSessionInternal) SetStdout(wr io.Writer) {
+	s.Session.Stdout = wr
+}
+
+// Set the stderr
+func (s sshSessionInternal) SetStderr(wr io.Writer) {
+	s.Session.Stderr = wr
+}
+
+// NewSession makes an sshSession from an sshClient
+func (s sshClientInternal) NewSession() (sshSession, error) {
+	session, err := s.srv.NewSession()
+	if err != nil {
+		return nil, err
+	}
+	return sshSessionInternal{Session: session}, nil
+}
+
+// Check interfaces
+var _ sshSession = sshSessionInternal{}