add support for media types components

[baywet]

OAS 3.2.0 defines a new component type, media types

On the component object, we should add the new property, and implement serialization/deserialization for it.

When serializing to OAS 3.1.0, a prefix of x-oai- should be used to make it an extension.

Additionally, the registration of media type components should be handled at the workspace level (see this and adjacent methods.

An interface IOpenApiMedia type should be abstracted away from the implementation type, references to the class should be updated to use the interface instead. And a new OpenAPIMediaTypeReference class should be introduced (see how it's done for other things like responses, parameters, etc...)

Lastly, the walkers should be updated to iterate over those components as well.