add FirstAsync in respository (#370)

* add FirstAsync in respository

* use new FirstOrDefaultAsync() method from repository

src/ApplicationCore/Interfaces/IAsyncRepository.cs

@@ -13,5 +13,7 @@ namespace Microsoft.eShopWeb.ApplicationCore.Interfaces
         Task UpdateAsync(T entity);
         Task DeleteAsync(T entity);
         Task<int> CountAsync(ISpecification<T> spec);
+        Task<T> FirstAsync(ISpecification<T> spec);
+        Task<T> FirstOrDefaultAsync(ISpecification<T> spec);
     }
 }