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.go

@@ -0,0 +1,73 @@
+//go:build !plan9
+// +build !plan9
+
+package sftp
+
+import "io"
+
+// Interfaces for ssh client and session implemented in ssh_internal.go and ssh_external.go
+
+// An interface for an ssh client to abstract over internal ssh library and external binary
+type sshClient interface {
+	// Wait blocks until the connection has shut down, and returns the
+	// error causing the shutdown.
+	Wait() error
+
+	// SendKeepAlive sends a keepalive message to keep the connection open
+	SendKeepAlive()
+
+	// Close the connection
+	Close() error
+
+	// NewSession opens a new sshSession for this sshClient. (A
+	// session is a remote execution of a program.)
+	NewSession() (sshSession, error)
+
+	// CanReuse indicates if this client can be reused
+	CanReuse() bool
+}
+
+// An interface for an ssh session to abstract over internal ssh library and external binary
+type sshSession interface {
+	// Setenv sets an environment variable that will be applied to any
+	// command executed by Shell or Run.
+	Setenv(name, value string) error
+
+	// Start runs cmd on the remote host. Typically, the remote
+	// server passes cmd to the shell for interpretation.
+	// A Session only accepts one call to Run, Start or Shell.
+	Start(cmd string) error
+
+	// StdinPipe returns a pipe that will be connected to the
+	// remote command's standard input when the command starts.
+	StdinPipe() (io.WriteCloser, error)
+
+	// StdoutPipe returns a pipe that will be connected to the
+	// remote command's standard output when the command starts.
+	// There is a fixed amount of buffering that is shared between
+	// stdout and stderr streams. If the StdoutPipe reader is
+	// not serviced fast enough it may eventually cause the
+	// remote command to block.
+	StdoutPipe() (io.Reader, error)
+
+	// RequestSubsystem requests the association of a subsystem
+	// with the session on the remote host. A subsystem is a
+	// predefined command that runs in the background when the ssh
+	// session is initiated
+	RequestSubsystem(subsystem string) error
+
+	// Run runs cmd on the remote host. Typically, the remote
+	// server passes cmd to the shell for interpretation.
+	// A Session only accepts one call to Run, Start, Shell, Output,
+	// or CombinedOutput.
+	Run(cmd string) error
+
+	// Close the session
+	Close() error
+
+	// Set the stdout
+	SetStdout(io.Writer)
+
+	// Set the stderr
+	SetStderr(io.Writer)
+}