Add feature flags & experiments API

[imjoehaines]

Goal

This PR adds the feature flags & experiments API to Bugsnag PHP. The API exists in the Client, Configuration and Report classes, which implement the new FeatureDataStore interface:

interface FeatureDataStore
{
    public function addFeatureFlag($name, $variant = null);
    public function addFeatureFlags(array $featureFlags);
    public function clearFeatureFlag($name);
    public function clearFeatureFlags();
}

Feature flags are stored on both the Configuration and Report (the Client forwards method calls on to the Configuration), allowing feature flags to be added globally (via Configuration) or to individual reports as required