feat: support multiple content type schemas per request body

[vivedo]

Handle different schemas per content type specified in the request body after Fastify added support for it in fastify/fastify#5545

Specifying more than one schema generated an incorrect OpenAPI spec:

{
  // ...
  "requestBody":{
    "content":{
      "application/json":{
        "schema":{
          "content":{
            "application/json":{
              "schema": {
                // ...
              }
            },
            "application/xml":{
              "schema": {
                // ...
              }
            }
            // ...
          }
        }
      }
    }
  }
}

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• commit message and code follows the Developer's Certification of Origin and the Code of conduct

[mcollina]

lgtm