multi: add TLV awareness to htlcswitch, pass extra EOB to the invoice registry

In this commit, we update the `HopIterator` to gain awareness of the new
TLV hop payload. The default `HopIterator` will now hide the details of
the TLV from the caller, and return the same `ForwardingInfo` struct in
a uniform manner. We also add a new method: `ExtraOnionBlob` to allow
the caller to obtain the raw EOB (the serialized TLV stream) to pass
around.

Within the link, we'll now pass the EOB information into the invoice
registry. This allows the registry to parse out any additional
information from the EOB that it needs to settle the payment, such as a
preimage shard in the AMP case.

htlcswitch/interfaces.go

@@ -23,10 +23,13 @@ type InvoiceDatabase interface {
 	// invoice is a debug invoice, then this method is a noop as debug
 	// invoices are never fully settled. The return value describes how the
 	// htlc should be resolved. If the htlc cannot be resolved immediately,
-	// the resolution is sent on the passed in hodlChan later.
+	// the resolution is sent on the passed in hodlChan later. The eob
+	// field passes the entire onion hop payload into the invoice registry
+	// for decoding purposes.
 	NotifyExitHopHtlc(payHash lntypes.Hash, paidAmount lnwire.MilliSatoshi,
 		expiry uint32, currentHeight int32,
-		hodlChan chan<- interface{}) (*invoices.HodlEvent, error)
+		hodlChan chan<- interface{},
+		eob []byte) (*invoices.HodlEvent, error)
 
 	// CancelInvoice attempts to cancel the invoice corresponding to the
 	// passed payment hash.