API: add support for fetching assign response by decision point/experiment

[bcb37]

To reduce unnecessary transfer of large amounts of data from assign calls, we should support returning only assigned conditions for supplied decision points or experiments.

• Create a new endpoint (or add a flag in the /assign endpoint) that contains experiment and/or decision point information in the request, which returns only the assigned conditions for that decision point or experiment.
• Update the client libraries to use that endpoint to fetch assignment data when the methods that return assignments for a decision point are called and there are no conditions for that decision point already in memory.