This commit is purely a refactor. In it, we let the `BlindedEdge` struct
carry a pointer to the `BlindedPayment` that it was derived from. This
is done now because later on in the PR series, we will need more
information about the `BlindedPayment` that an edge was derived from.
Since we now pass in the whole BlindedPayment, we swap out the
`cipherText` member for a `hopIndex` member so that we dont carry around
two sources of truth in the same struct.
A new interface AdditionalEdge is introduced which allows us to
enhance the CachedEdgePolicy with additional information. We
currently only need a PayloadSize function which is then used to
determine the exact payload size when building a route to adhere
to the sphinx package size limit (BOLT04).