Option to rename `requestBody`

In the OpenAPI v3 spec, the requestBody does not have a name. By default, it will be passed into the generated function as requestBody.

It would be nice to accept the commonly used x-body-name property in the operation schema to override this name.

Example:

paths:
  /foo:
    post:
      operationId: api.foo_get
      requestBody:
        x-body-name: payload
        content:
          application/json:
            schema:
              ...

# default
body: requestBody,

# based on x-body-name
body: payload,