Support private routing hints

[ruifm]

According the lnd rest api docs for AddInvoice it seems to be as easy as adding private=true in the rest api request.

This is currently not being done:

SatSale/node/lnd.py

Lines 122 to 136 in 752215f

	def create_lnd_invoice(
	self,
	btc_amount: float,
	memo: str = None,
	description_hash: str = None,
	expiry: int = 3600,
	) -> Tuple[str, str]:
	# Multiplying by 10^8 to convert to satoshi units
	sats_amount = int(float(btc_amount) * 10 ** 8)
	res = self.lnd.add_invoice(
	value=sats_amount, memo=memo, description_hash=description_hash, expiry=expiry
	)
	lnd_invoice = json.loads(MessageToJson(res))
	return lnd_invoice["paymentRequest"], lnd_invoice["rHash"]

Maybe it could be behind a config option.