Port `charge_number` to `Zuora::Local::RatePlanCharge`

Problem

In order to be able to use the local (Zuora::Local::RatePlanCharge) or remote (Zuora::Remote::RatePlanCharge) resource objects interchangeably based on a feature flag, we need to ensure these objects resemble each other as close as possible. This means that any attribute or method available to one class (the remote class) should be available to the other (the local class).

The Zuora::Remote::RatePlanCharge has an instance method defined charge_number. This method will need to be defined on the local resource. However, it may not need to be ported at all given the charge_number already exists as an attribute on the zuora_rate_plan_charges DB table.

Proposal

Let's determine if the charge_number method needs to be ported to Zuora::Local::RatePlanCharge. This method has some special logic defined on the remote resource, but this may not be applicable to the local resource. If determined this isn't needed, we can close this issue.