Policy based on exposure window

### Current Behavior

We want to manage and track SLAs - e.g. critical vulnerabilities must be fixed in X days using dependency track. Policy sounds like a good way to do that - define a policy based on exposure window, e.g. time since the policy was introduce in days. This is not available right now.

### Proposed Behavior

Pretty much what described above.

### Checklist

- [X] I have read and understand the [contributing guidelines](https://github.com/DependencyTrack/dependency-track/blob/master/CONTRIBUTING.md#filing-issues)
- [X] I have checked the [existing issues](https://github.com/DependencyTrack/dependency-track/issues) for whether this enhancement was already requested