feat(openapi): allow optional request body content

[monitaurus]

Q	A
Branch?	main
Tickets	Closes #2680
License	MIT
Doc PR	already existing (see doc)

Allow to generate a default request content or description if not defined in the extension of OpenApi using Model\RequestBody.

For example, on a POST /greetings/test operation on a Greeting entity with a id and a name, the following:

#[ApiResource(
    mercure: true,
    operations: [
        new Post(
            uriTemplate: '/greetings/test',
            openapi: new Operation(
                requestBody: new RequestBody(
                    description: 'Extended description',
                ),
            ),
        ),
    ],
)]

Will generate the following OpenApi, with the default generated content: