Define a OpenAPI request.
# Examples
```rust
use poem_openapi::{
payload::{Json, PlainText},
ApiRequest, Object,
};
#[derive(Object)]
struct Pet {
id: String,
name: String,
}
#[derive(ApiRequest)]
enum CreatePet {
/// This request receives a pet in JSON format(application/json).
CreateByJSON(Json<Pet>),
/// This request receives a pet in text format(text/plain).
CreateByPlainText(PlainText<String>),
}
```