Geo: PoC Replicators

What does this MR do?

Few improvements on @mkozono PoC for the self-service framework (!20672 (closed)) related to #35540 (closed).

My goal here is to have a centralized place where we describe the observed component that will have an "event" replicated.

Progress:

• Replicator class and app folder structure
• Event DSL inside replicator
• "Publish" interface
• Refactor existing code to use a replicator and the new "Publish" interface
• Merge multiple EventStore classes into publish_eventname on replicator
• Model DSL to associate a replicator and inject useful methods in it

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Usage draft

class Project < ActiveRecord::Base
  include Gitlab::Geo::Replicator::ModelIntegration

  with_geo_replicator ProjectReplicator

  after_save ->(model) { replicator.publish :created }
  after_destroy ->(model) { replicator.publish :deleted }
end