feat(account): add available countries bloc event

- Create new AvailableCountriesEvent abstract class
- Add FetchAvailableCountries event for fetching available countries list

Author: fulleni

lib/account/bloc/available_countries_event.dart

@@ -0,0 +1,12 @@
+part of 'available_countries_bloc.dart';
+
+abstract class AvailableCountriesEvent extends Equatable {
+  const AvailableCountriesEvent();
+
+  @override
+  List<Object> get props => [];
+}
+
+class FetchAvailableCountries extends AvailableCountriesEvent {
+  const FetchAvailableCountries();
+}