Use proxy references to resolve references

[MaggieKimani1]

This PR:

• Proposes the use of proxy reference objects to resolve both local and external references present in an OpenAPI document. A proxy reference class inherits the properties and methods of the class it proxies and is used to lazily retrieve the reference property values of its target.
• The proxy reference models use a host OpenApi document instance passed in their constructor in order to find the target the $refs point to and dereferences them lazily.
• Initially, we had a 2-step process where we load up a document in memory then have a second pass where we walk through the document and use an OpenApiReferenceResolver visitor to locate and resolve references. With this approach, the second pass includes walking through the document instance and using a HostDocumentResolver visitor that visits all proxy reference objects and sets the value of the host document.

To note:

• If one tries to parse a document fragment, they'll need to provide a host document value otherwise reference resolution fails.

Fixes #1571