Feature: Add ability to use request as an actor

What does this MR do and why?

Feature: Add ability to use request as an actor

Within the context of a "request" (as defined by SafeRequestStore.ensure_request_store), we add a new Flipper actor that will return the same flipper_id for that request.

## bad
Feature.enable_percentage_of_time(:new_feature, 10)
Feature.enabled?(:new_feature)
Feature.enabled?(:new_feature)  # <=== Not consistent

## good
Feature.enable_percentage_of_actors(:new_feature, 10)
Feature.enabled?(:new_feature, Feature.current_request)
Feature.enabled?(:new_feature, Feature.current_request)

Related issue: #425202 (closed)

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