multi: extract path ID and total amt from received payment

We've covered all the logic for building a blinded path to ourselves and
putting that into an invoice - so now we start preparing to actually be
able to recognise the incoming payment as one from a blinded path we
created.

The incoming update_add_htlc will have an `encrypted_recipient_data`
blob for us that we would have put in the original invoice. From this we
extract the PathID which we wrote. We consider this the payment address
and we use this to derive the associated invoice location.

Blinded path payments will not include MPP records, so the payment
address and total payment amount must be gleaned from the pathID and new
totalAmtMsat onion field respectively.

This commit only covers the final hop payload of a hop in a blinded
path. Dummy hops will be handled in the following commit.

htlcswitch/link.go

@@ -3774,9 +3774,6 @@ func (l *channelLink) sendHTLCError(pd *lnwallet.PaymentDescriptor,
 // that we're not part of a blinded route and an error encrypter that'll be
 // used if we are the introduction node and need to present an error as if
 // we're the failing party.
-//
-// Note: this function does not yet handle special error cases for receiving
-// nodes in blinded paths, as LND does not support blinded receives.
 func (l *channelLink) sendIncomingHTLCFailureMsg(htlcIndex uint64,
 	e hop.ErrorEncrypter,
 	originalFailure lnwire.OpaqueReason) error {