audit-scanner - MVP Stage 5.2 - reuse results from previous evaluations

[flavio]

Leverage previous PolicyReport objects as a cache for evaluations
Note: this can be done in parallel of stage 5.1

Acceptance criteria:

• See step 5 of audit RFC
• We have unit tests
• We have integration test if needed

[jvanz]

I'm moving this to block to wait for a @viccuad review.