Use proxy reference objects in place of `OpenApiReference` objects when models have references

[irvinesunday]

Related to: #1555

### Tasks
- [x] Use proxy references during deserialization of models where models have references
- [x] Validate models with proxy references as references and not as model classes.
- [x] Properly serialize models which have proxy references objects as their `OpenApiReference` if `InlineReferences` is false
- [x] Pass external file paths as an external resource to the proxy for external $refs resolution
- [x] Support inlining of Json schema $refs during serialization
- [x] Figure out how to pass a complete host document to proxy objects during parsing for ref resolution to work
- [x] Fix failing tests

### Tasks