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

fstest/testserver/init.d/TestSFTPRcloneSSH

@@ -0,0 +1,19 @@
+#!/bin/bash
+
+set -e
+
+# No password to make working with ssh binary easy
+
+NAME=rclone-serve-sftp-ssh
+IP=127.0.0.1
+PORT=28623
+
+start() {
+    run rclone serve sftp --addr ${IP}:${PORT} ${DATADIR}
+    
+    echo type=sftp
+    echo ssh=ssh -o StrictHostKeyChecking=no -p ${PORT} user@${IP}
+    echo _connect=${IP}:${PORT}
+}
+
+. $(dirname "$0")/rclone-serve.bash