Attachment API should include DELETE #408

SDK you're using (please complete the following information):

• Version 10.1.0

Is your feature request related to a problem? Please describe.
Currently if I want to delete an attachment, I need to dissociate the File. The FileApi is currently bugged and requires a workaround.

Describe the solution you'd like
Add a DELETE attachment for Invoice/Quote/Purchase Order etc.

Describe alternatives you've considered
Do a workaround to override the ApiClient base path allows one to use the FileApi for now.

Additional context
Add any other context or screenshots about the feature request here.