Current behavior of the GET api is that the response will include all "versions" of contracts matching the parameters, regardless of if endDate is set or not.  We also want to support the ability to only return "current" or "active" contracts.  To achieve this, we should be able to just add endDate as an optional query parameter for the API, and propagate it down to the ContractRepository method that queries the database.

More information can be found from 2/27 dev sync notes

https://docs.google.com/document/d/16bGEilEsp2nW2vVK2p9GEXdGOQyjvfOxpOtaN6Nfydg/edit