Introduce Redirect_Code class

[GaryJones]

Add a class that holds the enum list (class constants) of supported redirect HTTP status codes - like 301 and 302. Note that everything other than 301 is not yet used out of the box, but there is a filter around a 301; adding a constant for 302, for example, would allow someone to use this as well to set a 302 redirect.

Something like:

<?php

declare(strict_types=1);

namespace Automattic\LegacyRedirector\Domain\Redirect;

class Redirect_Code {
	public const MULTIPLE_CHOICES   = 300;
	public const MOVED_PERMANENTLY  = 301;
	public const FOUND              = 302;
	public const SEE_OTHER          = 303;
	public const USE_PROXY          = 305;
	public const TEMPORARY_REDIRECT = 307;
	public const PERMANENT_REDIRECT = 308;

	public static function valid_redirection_codes() {
		return [
			self::MULTIPLE_CHOICES,
			self::MOVED_PERMANENTLY,
			self::FOUND,
			self::SEE_OTHER,
			self::USE_PROXY,
			self::TEMPORARY_REDIRECT,
			self::PERMANENT_REDIRECT,
		];
	}
}