[POC] Use a default actor such as Request or correlation_id as global scope when feature flags cannot be scoped to actors

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As a follow-up to #327117 (comment 551751108), this issue to explore whether this is feasible and implement a POC:

I wonder if we actually need to retain "percentage of time", this will always be error prone and inaccurate, see how it is implemented in Flipper.

But we can turn almost anything into an actor through FeatureGate:

module FeatureGate
  def flipper_id
    return if new_record?
    "#{self.class.name}:#{id}"
  end
end

Because we can use an arbitrary number or string ➡️ https://github.com/jnunemaker/flipper/blob/a91863d6c5b7c79ac80868b3e5359a93b1c9d481/lib/flipper/gates/percentage_of_actors.rb#L32 it should be fairly easy to make a request / correlation_id an actor too, to make it less error prone.