Add a method to download an attachment from the observableendpoint class and be able to retrieve a file from a given observable.
The download can also be done with a zip and the parameter as_zip.

Renaming of the variable attachment_path to observable_path to be better align with the existing name conventions.

Reindent code in order to have lines that have less than 88 characters in order to pass the CI check

Remove space and add a double quote in order to fix flake8 and validate CI