Implement Export scan results and Download exported scan for WAS v2

[fullofentropy]

Is your feature request related to a problem? Please describe.
See Title: API features are not yet implemented and I would like them.
https://developer.tenable.com/reference/was-v2-scans-export
https://developer.tenable.com/reference/was-v2-scans-download-export

Describe the solution you'd like
Fork this repo and implement these API endpoints per the spec and do a pull request.
Plan: rework : ./tenable/io/scans.py->ScansAPI>export()

• add additional scan_type string/flag 'web-app2' in order to flag WASv2 scan
• refactor add logic to ensure this doesn't mess up what already exists
• add conditional to direct code flow if 'web-app2' is passed as 'scan_type'
• mirror a similar flow/style to what is already implemented for code readability.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.
waiting until someone else implements WAS v2 API specifications:
https://developer.tenable.com/reference/was-v2-scans-export

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