Saving to proto data store mechanism (not optimal)

[ricardoliveira5ro]

When saving one expense to the datastore, the current logic implemented is saving/overriding the entire list instead of appending the new expense in the file.
The same happens when updating one expense, it should update the existing expense. This one should be more tricky because it will probably require to delete the existing and creating a new one. In the current logic implemented the expense is being updated in the list before saving the entire list.