Update the `Subscriptions::RatePlanBuilder` to use local cache
Problem
The Subscriptions::RatePlanBuilder
currently makes a request directly to Zuora using the SubscriptionsByKey REST API. Instead of making this Zuora request, we could use the local Subscription data instead.
Proposal
Modify the Subscriptions::RatePlanBuilder
service to use the Zuora::Local::*
models instead of making the request to Zuora, if the feature flag is enabled. We could create a feature flag specific for this feature (e.g. zuora_subscription_cache_rate_plan_builder
) or re-use the flag we're already using in Zuora::Subscription
. Re-using the existing flag may be preferable.