`TIOEndpoint._wait_for_download()` now takes keyword parameters that it
passes through to the `_api.get()` call.

This makes it generally useful if we need to pass query parameters or
other web request modifiers in.

Pass through the scan type to allow the exporting of WAS data.
Named the parameter `scan_type` to make it a little clearer than the
raw API param: https://developer.tenable.com/reference#was-scans-export-status

NB. as this process makes three separate web calls we use two dicts of
parameters to ensure they don't mix.

Enable export of web-app scans