Feature: subscribe_on_quantized/subscribe_on_beat operator

Observables can express "tasks" or "jobs", that are continuously executing something, until they are either done, errored, or cancelled, and they do not necessarily emit anything beside the terminal signals as status.

To provide the ability for subscriptions to be made precisely at a quantized point in time, an operator very similar to the regular `subscribe_on` is required.

- Note that this may be more convenient to implement by having a dedicated quantized_task at the scheduler level, instead of just using a continuous task as is.
- Note that Schedule Tasks may have to be renamed to "Jobs" to not confuse the concept of an "observable task", for which the word "task" is more suited