Small enhancement - additional option to specify headers

Added requestHeaders map to be able to specify additional headers. Can be used if apidocs url is protected by basicAuth or setting custom url to generated openapi spec json|yaml
```
     requestHeaders = [
       "x-forwarded-host": "custom-host",
       "x-forwarded-port": "7000"
    ]
```

Ref feature https://github.com/springdoc/springdoc-openapi-maven-plugin