.Net: Review public API surface of OpenAPI plugins

[SergeyMenshykh]

Ensure that the public API surface of OpenAPI plugins is as small as possible and:

• All classes that need to be sealed and internal are sealed and internal.
• All public classes belong to the correct namespace and have internal constructors.
• Abstract classes are used instead of interfaces where it makes sense.
• All class members that should not be public are not public.
• Etc.

[SergeyMenshykh]

Consider exposing request and response headers from RestApiOperationResponse class public API surfce and remove all properties that came from the headers. This can be done before or after graduation.