Enable rescanning using the scan history diff endpoint

[allisonking]

Is your feature request related to a specific problem?

If a user has already done a scan before (there is something in the scan history), then we should render only the added systems in the registration flow. These systems should still go through the classify flow the same way as before.

Describe the solution you'd like

• Check if this is the first scan (check if GET /api/v1/plus/scan/latest 404s). Assuming it does not 404...
• After our normal PUT /api/v1/plus/scan, do a GET /api/v1/plus/scan/latest?diff=true to get the diff
• Render just the added_systems part of the response in the registration table
• Flow continues as normal

Describe alternatives you've considered, if any

A description of any alternative solutions or features you've considered.

Additional context

Requires https://github.com/ethyca/fidesctl-plus/pull/333.