Remove all evidence before a given date

## Overview

We have requirements on data retention, that once met should result in older data being expunged from the locker. There are manual ways to do this, but it seems like a nice use case for Prune to address, with associated tombstoning etc. Ideally we'd present some form of policy-as-json, such that different parts of the locker could be pruned at different cadences.

## Requirements

TBD

## Approach

TBD

## Security and Privacy

N/A

## Test Plan

TBD