Define a range of response codes

[patrickariel]

This makes it possible to define a range of response codes, like this:

use poem::http::StatusCode;
use poem_openapi::{payload::PlainText, ApiResponse};

#[derive(ApiResponse)]
enum CreateUserResponse {
    #[oai(status_range = "2XX")]
    Ok(StatusCode, PlainText<String>),
    #[oai(status_range = "4XX")]
    ClientError(StatusCode),
    #[oai(status_range = "5XX")]
    ServerError(StatusCode),
}

The inner StatusCode will be used to set the status code of the actual response.

The range syntax is defined in the official specification as follows:

To define a range of response codes, this field MAY contain the uppercase wildcard character X. For example, 2XX represents all response codes between 200 and 299. Only the following range definitions are allowed: 1XX, 2XX, 3XX, 4XX, and 5XX.

Closes #965.