lnrpc: include payment addr in main Invoice proto

With this change ListInvoices will return the payment addr, and the
response to AddInvoice will as well.

rpcserver.go

@@ -4766,6 +4766,7 @@ func (r *rpcServer) AddInvoice(ctx context.Context,
 		AddIndex:       dbInvoice.AddIndex,
 		PaymentRequest: string(dbInvoice.PaymentRequest),
 		RHash:          hash[:],
+		PaymentAddr:    dbInvoice.Terms.PaymentAddr[:],
 	}, nil
 }