feat: implement periodic/looping tasks ♻️

[robertrossmann]

Closes #5

This is an alternative implementation of what @CermakM proposed in #5 that retains the previous functionality (one-off task executions, hooks/observer methods, etc.) and adds a periodic/looping tasks on top of that.

Some implementation notes:

• the tasks are restarted immediately when the previous function's iteration returns.
• if the task returns an error, retry policies are applied just like for one-off tasks. Retries happen within the same loop iteration.
• if the retries are not successful, the observer's OnTaskFailed() method is called and a new loop starts again.
• the OnTaskStalled() and OnTaskSucceeded() methods are ignored for loop tasks.