Provide ability to copy OpenApiSchema

[Lanayx]

Is your feature request related to a problem? Please describe.
When creating schema transformer it's often required to create new schema by copying existing one (example). Currently such functionality is only available internally

Describe the solution you'd like
Either a new method should be added or internal changed to public

Describe alternatives you've considered
Copy source code in every project that needs that functionality

Additional context
I implemented custom transformer as a workaround for dotnet/aspnetcore#59528, since it's not going to be implemented any time soon