Add FOSSA scan to intersect

[leordev]

https://github.com/squareup/ospo/issues/163 involved some recon on how to add scans to intersect. This ticket is to actually do it. Follow the instructions in the Intersect README to add a new scan and PR it. Will likely need to work with Alex or #proj-simplify-code-scanning to put a FOSSA token in secretsmanager where Intersect seems to store it's other API keys.