Check planted evidence

## Overview

As evidence can be placed in the locker with [plant](https://github.com/ComplianceAsCode/auditree-plant) we should have some checks, beyond abandoned evidence, against that. I think something like warnings when within a certain time period of the evidence ttl & errors when within a shorter period would be a good start.

## Requirements

- Walk all the external evidence
- Warn if ttl is N days away (N from config, default 21 days?)
- Error if ttl is M days away (M from config, default 7 days?)

## Approach

_Provide a detailed approach to satisfy all of the requirements listed in the
previous section. This level of detail may not be available at the time of
issue creation and can be completed at a later time._

## Security and Privacy

_Provide the impact on security and privacy as it relates to the completion of
this issue. This level of detail may not be available at the time of
issue creation and can be completed at a later time. N/A if not applicable._

## Test Plan

_Provide the test process that will be followed to adequately verify that the
approach above satisfies the requirements provided. This level of detail may
not be available at the time of issue creation and can be completed at a later
time._

No implementation files, tests, or entry points are named. Start by locating the external-evidence checks and configuration entry points, then trace how planted evidence is discovered and how check results report warnings and errors. Done means all external evidence is checked with configurable warning and error windows, defaulting to 21 and 7 days.

Written by the indexing model from the issue text.