func (ln *Client) GetPrivateKey() (sk *btcec.PrivateKey, err error) {
	return ln.GetCustomKey(0, "nodeid")
}
GetPrivateKey gets the custom key with the parameters that return the node's master key (0 and "nodeid")
package lightning
import (
	"crypto/sha256"
	"errors"
	"io"
	"io/ioutil"
	"path/filepath"
	"github.com/btcsuite/btcd/btcec"
	"golang.org/x/crypto/hkdf"
)