When we have a HTLC that is part of a blinded route, we need to include
the next ephemeral blinding point in UpdateAddHtlc for the next hop. The
way that we handle the addition of this key is the same for introduction
nodes and relaying nodes within the route.
This commit introduces a blinding kits which abstracts over the
operations required to decrypt, deserialize and reconstruct forwarding
data from an encrypted blob of data included for nodes in blinded
routes.
Update test to include the sphinx action to more closely represent
reality. This will be required when we add more validation to the
presence of a nextChanID field. A MoreHops action is chose because
we're testing the case with a payload that contains forwarding info.
