Add support for loading feature flags & experiments in configuration

[imjoehaines]

Goal

This PR adds support for loading feature flags & experiments in the bugsnag.yaml config file, e.g.:

bugsnag:
    feature_flags:
        - name: Checkout button color
          variant: Blue
        - name: Special offer
          variant: Free coffee
        - name: New checkout flow

This is useful when features are known upfront, but when that's not possible the feature flags API can be used to configure feature flags at runtime

I've also added Maze Runner tests for feature flags in our Symfony fixture apps