Bring back Notifications support with new Scheduler

[Supereg]

Problem

#44 introduced a completely new version of SpeziScheduler. This version currently didn't introduce out of the box support for notifications as it was previously supported.

Solution

We found, that the previous SpeziScheduler API exposed some generic notifications API that might have been better but in a more central Spezi package. The goal of the initial #44 PR was to introduce the new Scheduler model. A follow-up PR should be used to investigate how to best integrate notification support with the new Scheduler and evaluate if there is possibility for reuse across the Spezi framework ecosystem.

Additional context

We should investigate what support of backwards compatibility is necessary to make sure the previously scheduled notifications from previous Scheduler versions are automatically removed.

Code of Conduct

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