Refactor/dedupe the command/alternate-command execution code in plugin and ucp package

[prkalle]

Describe the feature request
The implementation code for command/alternate-command execution existing in plugin(sync_plugin.go) and ucp(ucp.go) package should be refactored and avoid duplicate code.

const (
	// customCommandName is the name of the command expected to be implemented
	// by the CLI should there be a need to discover and alternative invocation
	// method
	customCommandName string = "_custom_command"
)

// cmdOptions specifies the command options
type cmdOptions struct {
	outWriter io.Writer
	errWriter io.Writer
}

type CommandOptions func(o *cmdOptions)

// WithOutputWriter specifies the CommandOption for configuring Stdout
func WithOutputWriter(outWriter io.Writer) CommandOptions {
	return func(o *cmdOptions) {
		o.outWriter = outWriter
	}
}

// WithErrorWriter specifies the CommandOption for configuring Stderr
func WithErrorWriter(errWriter io.Writer) CommandOptions {
	return func(o *cmdOptions) {
		o.errWriter = errWriter
	}
}

// WithNoStdout specifies to ignore stdout
func WithNoStdout() CommandOptions {
	return func(o *cmdOptions) {
		o.outWriter = io.Discard
	}
}

// WithNoStderr specifies to ignore stderr
func WithNoStderr() CommandOptions {
	return func(o *cmdOptions) {
		o.errWriter = io.Discard
	}
}

func runCommand(commandPath string, args []string, opts *cmdOptions) (bytes.Buffer, bytes.Buffer, error) {
	command := exec.Command(commandPath, args...)

	var stderr bytes.Buffer
	var stdout bytes.Buffer

	wout := io.MultiWriter(&stdout, os.Stdout)
	werr := io.MultiWriter(&stderr, os.Stderr)

	if opts.outWriter != nil {
		wout = io.MultiWriter(&stdout, opts.outWriter)
	}
	if opts.errWriter != nil {
		werr = io.MultiWriter(&stderr, opts.errWriter)
	}

	command.Stdout = wout
	command.Stderr = werr

	return stdout, stderr, command.Run()
}

Describe alternative(s) you've considered

Additional context

Reference: #106 (comment)