Create a mixin for all methods common to local and remote RatePlanCharge classes

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).

Proposal

Let's move all common instance and class methods to a new mixin module that can be included in both Rate Plan Charge classes, Zuora::Remote::RatePlanCharge and Zuora::Local::RatePlanCharge. This would involve any method that does not require any customization like price for example.