Add support for generation of ApiManifest from an OpenApi document instance

[peombwa]

The library should provide an extension method that generates an ApiManifest from an OpenApi document instance.

The extension method will be used by downstream tools to generate an instance of an ApiManifest document from an OpenAPi document.

• Provide document showing the mapping of OpenApi fields to ApiManifest fields.
• Provide ToApiManifest extension method of OpenApiDocument.

This is needed in support of microsoft/OpenAPI.NET#1384.