Support read- and write only fields

[rafalkrupinski]

Is your feature request related to a problem? Please describe.
non-nullable read- and write-only fields are required only in either request or response objects. In the generated client they are all required tho.

Describe the solution you'd like
A simple solution would be to make all required read and write-only fields behave as nullable.
Another one would be introduce a base model class with all other fields and writeable & readable subclasses with write and read-only fields respectively.

Additional context
I'd be happy to prepare a PR for the first approach

[dbanty]

#672 will serve as the new discussion for this (moving features to discussions and keeping issues only for bugs)