[REQ] Specify native class via vendor extension

Is your feature request related to a problem? Please describe.

I know you can use type mappings, but it'd be great if I could define the type something should map too directly in my OpenAPI spec, similarly to x-implements.

Describe the solution you'd like

Something like x-implements, e.g. x-type or something like that.

Describe alternatives you've considered

Additional context

My workflow involves using typespec to generate the OpenAPI spec and I have some general models inside a library I reuse, having to specify the type mapping "in" the generator means I have to do this in each separate project where I use my library, and remember to update those whenever I add a new type inside said library.

Start by reading how OpenAPI Generator currently handles type mappings and the existing x-implements vendor extension. Compare that behavior with the TypeSpec-generated OpenAPI workflow described in the issue, then determine the extension's intended scope across generators. Done means a documented, consistently supported way to declare the target native type directly in the OpenAPI specification.