Feature: Model validation including xp fields

[crhistianramirez]

Any kind of model validation on the OrderCloud model such as Required, MaxLength, etc. should be applied to models in the SDK as well, and similarly there should be a way to define if xp is required if any xp properties are also required or have some other validation associated with them.

[tmenier]

WIP. Would like to get it working with partials/PATCH before releasing.