Update requirements system to allow increment rollout of requirement changes

Problem

Currently our requirements system is single constant that allows for config of the requirements of a product. This is a problem when we want to rollout changes to environments incrementally (for testing or controlled release), as any change will go to all environments.

Proposal

1. Change the requirements definition to be a method instead of a constant, so that conditional configuration based on feature flags can be applied.
2. Refactor the requirements system to accept a purchase object instead of a raw hash so that we can validated based on all the purchase args more easily.

For example, a simple implementation of this might look like:

def self.check(purchase)
  requirements = case purchase.plan_type
                 when :gitlab_credits
                   gitlab_credits_requirements(purchase)
                 else
                   Failure()
                 end

  requirements.filter_map { |requirement| requirement.unmet? & requirement.error_message }
end

def self.gitlab_credits_requirements(purchase)
  requirements = []

  requirements << ActiveSubscription.new(subscription: purchase.subscription) if feature_flag_disabled?
  requirements << NewRatePlan.new(product_rate_plan_charge: purchase.product_rate_plan_charge, subscription: purchase.subscription)

  # ...

  requirements
end