Add feature flags module

[scottgerring]

To fill the gap with Powertools for Python, we should add a feature flags module. The feature flags utility provides a simple rule engine to define when one or multiple features should be enabled depending on the input. We will need to implement or integrate a rules engine in order to support the feature flag schema used in the python module.

Python version: https://awslabs.github.io/aws-lambda-powertools-python/2.4.0/utilities/feature_flags/#getting-all-enabled-features