Extend Reference functionality

[DiegoFaFe]

Motivation

Using the sdk I've noticed some code that we end up repeating pretty often around References, like having to create a new reference to request a language variant of a content item that I already have, or checking agaisnt Guid.Empty when I want to filter if the items are in the default language/workflow...

Proposed solution

• [ ] Add a static Default method to Reference class (will return a Reference instance with Guid.Empty id and default codename)
• [ ] Implement IEquatable to Reference class
• [ ] Add Implicit conversor from sdk models (ContentItemModel, LanguageVariantModel...) to Reference

Additional context

[DiegoFaFe]

@pokornyd I can prepare a PR with any of the 3 proposed changes if they are approved. Just let me know which ones you see value in