rpcserver: populate features on lnrpc invoices

In the process, we also move the feature serialization into the
invoicesrpc package, so that it can be shared between the invoicesrpc
and main rpcserver.

rpcserver.go

@@ -4632,20 +4632,6 @@ func (r *rpcServer) DecodePayReq(ctx context.Context,
 		paymentAddr = payReq.PaymentAddr[:]
 	}
 
-	// Convert any features on the payment request into a descriptive format
-	// for the rpc.
-	invFeatures := payReq.Features.Features()
-	features := make([]*lnrpc.Feature, 0, len(invFeatures))
-	for bit := range invFeatures {
-		name := payReq.Features.Name(bit)
-		features = append(features, &lnrpc.Feature{
-			Bit:        uint32(bit),
-			Name:       name,
-			IsRequired: bit.IsRequired(),
-			IsKnown:    name != "unknown",
-		})
-	}
-
 	dest := payReq.Destination.SerializeCompressed()
 	return &lnrpc.PayReq{
 		Destination:     hex.EncodeToString(dest),
@@ -4660,7 +4646,7 @@ func (r *rpcServer) DecodePayReq(ctx context.Context,
 		CltvExpiry:      int64(payReq.MinFinalCLTVExpiry()),
 		RouteHints:      routeHints,
 		PaymentAddr:     paymentAddr,
-		Features:        features,
+		Features:        invoicesrpc.CreateRPCFeatures(payReq.Features),
 	}, nil
 }