Implement a relationship plugin to allow data from multiple Fitbit endpoints to be combined

While it's nice to hit one endpoint at a time and get the data provided by that endpoint, and that's certainly useful, it should be possible to combine the data from multiple endpoints if the site admin so chooses. Implement a relationship plugin through which the site admin is given the power to spec out which Fitbit API endpoints they want data from. The implementation should only hit API endpoints which have been requested by configuring Relationships in the views UI.