Add A/B testing capabilities for calls

[glencbz]

We want to figure out what parameters can reduce the incidence of phone calls being dropped. To do so, we need to be able to run experiments on users where different calls use different parameters.

The easiest way to implement this is to store an 'experiment configuration' for each user and to expose it via the /features endpoint and FeatureService interface. When a user makes a call, it will use this configuration.