Introduce a checkout init event

Um, just a thought but would we expect to be emitting the event if, say, a commerce_recurring order in the needs_payment state is being put back through checkout so the customer can remedy a failing recurring payment? This was made possible by #3047103: Make it easier to use checkout with non-draft orders → .

I would think in that case it's a "no". Could we consider checking the order state is draft before emitting the event?

Updated patch #15 to support D10.3

before:
$this->eventDispatcher->dispatch(CheckoutEvents::INIT, $event);

after
$this->eventDispatcher->dispatch($event, CheckoutEvents::INIT);