Handle cumulative evidence

## Overview

Provide a way to more easily handle and/or mark a fetcher/evidence as one that's accumulating evidence on a timely basis. This will ensure that proper logic is added to ensure evidence continuity from one run to the next.

## Requirements

- The key thing is making it clear that a fetcher is collecting cumulative evidence - this is important at review time, for e.g. We could also provide some helpers, such as getting the last run date easily.
- TBD... Needs more research, thought, discussion

## Approach

- TBD
- May be a fetcher subclass??
- May be an evidence subclass??
- May be both

## Security and Privacy

N/A

## Test Plan

TBD