OpenApiServer does not maintain extensions on generation through CLI

[jvandertil]

TL;DR: According to the spec (https://swagger.io/specification/) the OpenApiServer object MAY be extended with Specification extensions. However the current implementation does not support that.

Would you accept a PR to enable this support?

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We are trying to migrate from Swashbuckle (which seems unmaintained) to NSwag. For an internal API platform we rely on extension properties being attached to (amongst others) the OpenApiServer object (https://github.com/RicoSuter/NSwag/blob/master/src/NSwag.Core/OpenApiServer.cs).

According to the spec (https://swagger.io/specification/) the OpenApiServer object MAY be extended with Specification extensions. However the current implementation does not support that.

We generate our swagger.{json,yaml} using the CLI generator which deserializes and (re-)serializes the original specification generated by our application, and the extensions we added are lost.

From a quick review it seems that extending the OpenApiServer class with JsonExtensionObject would solve this issue. If so, then I can provide a pull request for that.

I believe there are more types that according to the spec MAY be extended, but currently don't support extensions. But I haven't look into that too much.