The "path" is just a message to ourselves. This meets the minimal
requirement for bolt12 invoices: that there be a blinded path (at
least so we can use the path_id inside in place of "payment_secret").
We expose the method to make this path_id to a common routine: offers
will need this for generating more sophisticated paths.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
