Export schemas as interfaces instead of types

[Dominik Broj]

Our OpenAPI schemas are not fully correct due to annotations issues on the backend. At the same time I'd like to start http client from openapi-codegen. To make it easier and prevent doing changes in autogenerated files, I'd like to have a file with all the necessary customizations and augment autogenerated schemas. However, because schemas are defined as types instead of interfaces, it is not possible.

Would it be possible to make it configurable so that we have a flag that decides whether in exported schemas types or interfaces are used?