There is no way to indicate the requestBody is required

With current implementation, in case where there is request body for the API, the generated spec always presents that the request body is optional (which is not marked with required).

AS-IS
```yaml
paths:
/v1/authentication/request:
post:
requestBody:

....
```

TO-BE (if the request body is required)
```yaml
paths:
/v1/authentication/request:
post:
requestBody:
required: true
....
```

It seems that there is no way to mark required for `RequestFieldsSnippet`, such as optional() or required() method. Instead, we might leverage attributes, Map to indicate that the requestBody is required.

Is this reasonable?