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/sftp_test.go

@@ -30,3 +30,13 @@ func TestIntegration2(t *testing.T) {
 		NilObject:  (*sftp.Object)(nil),
 	})
 }
+
+func TestIntegration3(t *testing.T) {
+	if *fstest.RemoteName != "" {
+		t.Skip("skipping as -remote is set")
+	}
+	fstests.Run(t, &fstests.Opt{
+		RemoteName: "TestSFTPRcloneSSH:",
+		NilObject:  (*sftp.Object)(nil),
+	})
+}