[scheduler][primitive] Allow to drag events from outside

[flaviendelangle]

Developers should be able to render events outside of the Scheduler and then drag and drop them inside our various primitives.

In this PR:

• Create a new primitive (something like <DraggableEvent />)
• Make sure this primitive can be dropped in the Time Grid, the Day Grid and the Timeline
• Throw an error if the event already exists (matching id)

Not in this PR:

• Recurring event from the outside

[github-actions]

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