EventualBuilder provides a builder that rebuilds every time that any of
notifiers is updated. Either of them must be provided.
Extends EventualValue, so that subscribed Widgets can rebuild
upon changes on the current value, error events or loading status
EventualSingleBuilder provides up to four different builders that are used according to the
current state of
EventualValue<T> wraps and manages eventual data, which can be still unresolved,
have an error, or have a non-null valid value.