[Functionality] Implement `Example` trait for `ApiResponse` / `ResponseContent`

[lucemans]

Description of the feature

Implement Example trait for ApiResponse / ResponseContent

Code example (if possible)

In a similar maner to how with the Object macro you can specify

#[derive(Serialize, Deserialize, Object)]
#[oai(example)]
pub struct MyStruct {
    ...
}

You would be able to specify it for arbitrary (user defined) response types

#[derive(ResponseContent)]
#[oai(example)]
enum IcsContent {
    #[oai(content_type = "text/calendar")]
    Calendar(PlainText<String>),
}
    
#[derive(ApiResponse)]
enum CalendarResponse {
    #[oai(status = 200)]
    Ok(IcsContent),
}

const SAMPLE_ICS: &str = include_str!(".//sample.ics");

impl Example for IcsContent {
    fn example() -> Self {
        Self::Calendar(PlainText(SAMPLE_ICS.to_string()))
    }
}

[lucemans]

Currently am shimming the openapi.json file converting it to the following:

"/domains.ics": {
      "get": {
        "tags": [
          "Calendar"
        ],
        "responses": {
          "200": {
            "description": "",
            "content": {
              "text/calendar": {
                "schema": {
                  "type": "string"
                },
                "example": "BEGIN:VCALENDAR\nVERSION:2.0\nPRODID:ICALENDAR-RS\nCALSCALE:GREGORIAN\nBEGIN:VEVENT\nDTSTAMP:20250418T045417Z\nDESCRIPTION:example.com\\ngithub.com\nDTEND;VALUE=DATE:20260217\nDTSTART;VALUE=DATE:20260217\nSUMMARY:3 domains expire on 2026-02-17 00:00:00 UTC\nUID:5bc62a77-3fc6-4b2b-bd89-55190392d4e4\nEND:VEVENT\nEND:VCALENDAR\n"
              }
            }
          }
        }
      }
    }