Abstract trait for DataClient.

This type can be used by applications that want to mock DataClient in unit tests by passing parameters of type DataClientApi instead of accessing the DataClient singleton directly everywhere. For example:

// Get the DataClientApi passed to functions instead of them accessing the DataClient singleton.
def logicToTest(dataClient: DataClientApi, ...) = ???

object Application {
   // The application instantiates the real DataClient.
   val dataClient: DataClientApi = DataClient()
   logicToTest(dataClient, ...)
}

object Test {
   // Tests generate a mock or a stub from the trait.
   val dataClientStub: DataClientApi = stub[DataClientApi]
   logicToTest(dataClientStub, ...)
}