[Atoms] - Add `onTimeslotsLoaded` prop to `Booker` atom

[SomayChauhan]

We need to support an onTimeslotsLoaded callback in the Booker atom. This should be triggered once the available timeslots have been successfully fetched. It'll allow consumers to hook into that event—for example, for logging, analytics, or triggering related UI updates.

[linear]

CAL-5926 [Atoms] - Add `onTimeslotsLoaded` prop to `Booker` atom