Feature flag evaluation tracking integration for django-waffle

### Problem Statement

We use [django-waffle](https://pypi.org/project/django-waffle), which is a Django plugin, to store and use feature flags. We want to surface these in Sentry via the existing [Feature Flag Evaluation Tracking](https://docs.sentry.io/platforms/python/feature-flags/#enable-evaluation-tracking) feature.

### Solution Brainstorm

It would be most convenient to surface these in Sentry automatically via a built-in integration.

### Prioritisation

According to some back-of-the-envelope maths, 1 in 35 django projects ([50 million downloads per month](https://pypistats.org/packages/django)) use django-waffle ([1.4 million downloads per month](https://pypistats.org/packages/django-waffle)).