Add boundary time observer API

[defagos]

As for periodic time observers, we should add a boundary time observer API to let users conveniently register them with the controller after creation.

Unlike periodic time observers which are grouped depending on the time interval chosen (so that events occur at the same time), such a mechanism is not needed for boundary time observers. I would also check whether this grouping for periodic time observers is really a good idea and remove it if the answer is netagive.