Feature request: support for readOnly fields

Right now there is no direct support to mark a particular field as `readOnly`. While using the API first approach, we are right now using the same data model as an input and output of the API call, obviously to limit the duplication of definitions. We are using `readOnly` attribute to indicate the fields that are not meant to be set by the client application as the input fields, but we would like to have such fields present in the output (and set by the server app).

It would be nice to have support in this tool to mark a particular property of a class readonly, e.g. by using `System.ComponentModel.ReadOnlyAttribute` or similar (preferrably generic) way and use that to identify such field as `readOnly` in the output openapi schema.