Add Liquid SDK Event & Log stream handlers to example app (#305)

Update example app
* Use defaultConfig on ConnectRequest
* Update feesSat as non-null
* Use payment & walletInfo streams from BreezLiquidSDK

packages/flutter/example/lib/routes/home/widgets/bottom_app_bar.dart

@@ -49,7 +49,7 @@ class HomePageBottomAppBar extends StatelessWidget {
                   context: context,
                   builder: (context) => ReceivePaymentDialog(
                     liquidSDK: liquidSDK,
-                    paymentsStream: paymentsStream.asBroadcastStream(),
+                    paymentsStream: paymentsStream,
                   ),
                 );
               },

packages/flutter/example/lib/routes/home/widgets/payment_list/widgets/payment_item.dart

@@ -62,7 +62,7 @@ class PaymentItem extends StatelessWidget {
             "${item.paymentType == PaymentType.send ? "-" : "+"}${item.amountSat} sats",
             style: Theme.of(context).textTheme.bodyLarge,
           ),
-          if (item.feesSat != null) ...[
+          if (item.feesSat != BigInt.zero) ...[
             Text("FEE: ${item.paymentType == PaymentType.receive ? "-" : ""}${item.feesSat} sats"),
           ]
         ],