Make `requestBody` for create operation optional

### Use Cases or Problem Statement

In my case a creation of the resource has only a POST method without a `requestBody`. The object is generated randomly on the server side and returned in the response body.

I can see DESIGN.md specifies clearly, that create with request body _is the only schema required for resources_.

When trying to generate the code, the resource is skipped due to above requirement - missing `requestBody`.

### Proposal

To my understanding, as long as there's a schema body in either request or response the resource mapping shall work.

The proposal is to modify `resource_mapper::generateResourceSchema` function:
1. for create request body - in case of missing schema do not return error, but set createRequestAttributes to empty list
2. for create response body - leave as it is
3. merge two sets of attributes and return error if the merged set is empty.

### Additional Information

I have a limited knowledge about the context, it's probably not seeing a big picture / all implications. It would be great if you could explain the rationale behind this requirement if it is a hard requirement.

### Code of Conduct

- [X] I agree to follow this project's Code of Conduct