Support receiving values in the remote events API

There is no proper API to receive both keys and values from remote events without resorting to KeyValueConverters that produces custom events with keys and values together. Converters are currently used by Near Cache, Spark connector, JCache and also Spring session.
Ideally there should be a way to receive the event with keys and values separated, this facilitates transcoding (clients may choose to receive keys and values in different formats) and requires less efforts in the client to consume those events.