feat(ssh+cli): Support SSH CLI on machine init and add commands (#173)

* Add SSHCLIRemote type with buildSSHArgs

Implement's Executor interface (Run, Stream, Close)

* Add UseSSHCLI field to RemoteMachine struct

Allow us to differentiate regular ssh from ssh+cli.

* Parse ssh+cli:// scheme on machine init/add

* Support SSH CLI when provisioning

* Update docs to reflect ssh+cli:// support

* Revert removal of `init` examples

Fixes copy & pasta mistake when documenting the new ssh+cli connection
methods.

internal/sshexec/sshcli.go

@@ -0,0 +1,72 @@
+package sshexec
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"os/exec"
+	"strconv"
+	"strings"
+)
+
+type SSHCLIRemote struct {
+	user    string
+	host    string
+	port    int
+	keyPath string
+}
+
+func NewSSHCLIRemote(user, host string, port int, keyPath string) *SSHCLIRemote {
+	return &SSHCLIRemote{
+		user:    user,
+		host:    host,
+		port:    port,
+		keyPath: keyPath,
+	}
+}
+
+// TODO: Refactor and reuse this with buildDialArgs and buildSSHArgs from
+// SSHCLI Connector.
+func (r *SSHCLIRemote) buildSSHArgs() []string {
+	args := []string{"-o", "ConnectTimeout=5"}
+
+	if r.port != 0 && r.port != 22 {
+		args = append(args, "-p", strconv.Itoa(r.port))
+	}
+
+	if r.keyPath != "" {
+		args = append(args, "-i", r.keyPath)
+	}
+
+	args = append(args, r.user+"@"+r.host)
+	return args
+}
+
+func (r *SSHCLIRemote) Run(ctx context.Context, cmd string) (string, error) {
+	args := r.buildSSHArgs()
+	args = append(args, cmd)
+
+	execCmd := exec.CommandContext(ctx, "ssh", args...)
+	output, err := execCmd.CombinedOutput()
+	if err != nil {
+		return strings.TrimSpace(string(output)),
+			fmt.Errorf("run command on remote host: %w: %s", err, string(output))
+	}
+	return strings.TrimSpace(string(output)), nil
+}
+
+func (r *SSHCLIRemote) Stream(ctx context.Context, cmd string, stdout, stderr io.Writer) error {
+	args := r.buildSSHArgs()
+	args = append(args, cmd)
+
+	execCmd := exec.CommandContext(ctx, "ssh", args...)
+	execCmd.Stdout = stdout
+	execCmd.Stderr = stderr
+
+	return execCmd.Run()
+}
+
+// no-op as there is no persistent connection.
+func (r *SSHCLIRemote) Close() error {
+	return nil
+}