Add publish_group feature to Gitlab::EventStore

What does this MR do and why?

This MR adds functionality to publish group of events through the GitLab EventStore. Currently, when an event is published to EventStore, a worker job is invoked for the event. There are a few cases where we dispatch a lot of events of same type in a single business transaction. One example is, when a user is added to a group, we will dispatch ProjectAuthorizations::AuthorizationsChangedEvent for each of the projects within the group. This results in a number of sidekiq workers invoked at the same time thereby putting pressure on sidekiq.

With publish_group, we can dispatch a group of events of same type so that the subscriber worker receives a batch of events instead of single event. The group size could be configured using group_size parameter while creating subscription.

Addresses #424938 (closed)

Example

Subscription

store.subscribe ::Security::RefreshProjectPoliciesWorker, to: ::ProjectAuthorizations::AuthorizationsChangedEvent, group_size: 25

Publishing events

events = project_ids.map do |project_id|
  ProjectAuthorizations::AuthorizationsChangedEvent.new(data: { project_id: project_id })
end
Gitlab::EventStore.publish_group(events)

MR acceptance checklist

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