Add multiple events support to waitForEvent

[elbalexandre]

Is your feature request related to a problem? Please describe.
Waiting for multiple events (using step.waitForEvent) result in multiple function invocations.

Describe the solution you'd like
As we can send a batch of event, it could be suitable to be able to wait for a batch of event with a single request. The function waiting for theses event could be invoked a single time (after waitForEvent) when all the awaited event have been triggered.

api:

await step.waitForEvent(`wait-all-objects-sync-completed`, object.map(object => ({
  event: 'object/sync.completed',
  timeout: '1 day',
  if: `async.data.objectId == '${object.id}'`,
}))

Describe alternatives you've considered
N/A

Additional context
We are using waitForEvent for a variable number of event. This number can be large.

const eventsPromise = Promise.all(objects.map(object => step.waitForEvent(`wait-sync-completed-${object.id}`, {
  event: 'object/sync.completed',
  timeout: '1 day',
  if: `async.data.objectId == '${object.id}'`,
})))
 
// theses event will trigger 'object/sync.completed' after their completion
await step.sendEvent(
  'start-object-sync',
  objects.map(object => ({
    name: 'object/sync.requested',
    data: { objectId: object.id },
  }))
);
 
await eventsPromise;