lnrpc+rpcserver: encode custom records as custom channel data

With this commit we encode the custom records as a TLV stream into the
custom channel data field of the invoice HTLC.
This allows the custom data parser to parse those records and replace it
with human-readable JSON on the RPC interface.

lnrpc/invoicesrpc/utils.go

@@ -123,6 +123,16 @@ func CreateRPCInvoice(invoice *invoices.Invoice,
 			MppTotalAmtMsat: uint64(htlc.MppTotalAmt),
 		}
 
+		// The custom channel data is currently just the raw bytes of
+		// the encoded custom records.
+		customData, err := lnwire.CustomRecords(
+			htlc.CustomRecords,
+		).Serialize()
+		if err != nil {
+			return nil, err
+		}
+		rpcHtlc.CustomChannelData = customData
+
 		// Populate any fields relevant to AMP payments.
 		if htlc.AMP != nil {
 			rootShare := htlc.AMP.Record.RootShare()