Make the generation of `unknown` types for undefined spec configurable

[jmechevarria]

Description

This is similar, but not quite the same, to https://github.com/hey-api/openapi-ts/issues/820.

Spec excerpt:

paths:
  /login:
    post:
      operationId: loginMethod
      responses:
        200:
          description: OK
      requestBody:
        content:
          application/json:
            schema:
              $ref: "#/components/schemas/LoginRequest"

Generated TS relating to the response body:

// types.gen.ts
export type LoginMethodResponse = unknown;

In this case, if I don't define anything for the response body, it still gets generated as an unknown type. The name of this type is inferred from the operationId field, eg:

operationId: loginMethod -> LoginMethodResponse

This is unexpected behavior for me, after switching from openapi-typescript-codegen.

Still, it might be by design now, which is why I'm not reporting a bug, but instead asking for the possibility to make it configurable; thus allowing the developer to choose NOT to generate a type at all (even void would be preferable here).

Thanks